Issei has developed many relationships throughout the series, both within the Occult Research Club and the Three Factions. He has also attracted the attention of most of the female characters, despite his lecherous nature.
She is the woman who reincarnated Issei and is the love of his life. When she first summoned him, she treated him as a "special" case, knowing that he had a lot of potential. However, as time went on, she discovered his soft side and started to fall in love with him. She and Issei both shared their first kiss with each other following the latter's duel with Raiser. She often gets jealous when other girls flirt with or tempt him. She loves him very much and is willing to do anything with him. In response, Issei will do anything to protect her, as he loves her just as much as she loves him. One time, she even tried to have sex with him to get out of an arranged marriage. To add to that, Issei constantly wakes up with her sleeping next to him naked. For this reason, she says that she has no problem with letting Issei see her naked and even lets him touch her breasts from time to time, which she constantly says she has no problem with.
In Volume 10, Issei was given permission from Rias' mother to start a harem to keep the girls happy. She was completely heartbroken by the news of Issei's death (in which Sairaorg berates her for), but she gained a great deal of hope when she found that his spirit was still alive through his Evil Pieces. It also appears that when Issei sees her breasts or touches them, he gains a tremendous amount of power, which helped him achieve Balance Breaker and Trident, and also replenished his depleted stamina and energy. This, however, has resulted in her gaining derogatory nicknames such as the "Switch Princess" (Bikou) and the "Crimson Bust Princess" (Azazel). She is first in Issei's harem. At the end of Volume 10, Rias and Issei end up becoming a couple.
Subnote: Issei has been in love with Rias for just as long as she has been in love with him, but he could not tell her his feelings due to the trauma that he suffered from Raynare, the Fallen Angel, from killing him and because he rejected her plea for mercy just as Rias killed her.
Asia met Issei in Vol. 1 and he was lecherous as usual, going as far as calling her a "blonde-haired bishōjo". They meet each other again during a mission where he battled Freed Sellzen, whom she was working with after she left the Church. After the incident, she left Freed and was immediately entranced by Issei due to him wanting to protect her. She was kidnapped by Raynare and later died after the latter took her Sacred Gear. Issei avenges her and with the help of Rias, brings her back to life as a Devil, which then results in her falling in love with him.
Following the incident, Issei started to believe that he should always protect Asia, feeling that he would never want to go through another incident of losing her again. He often has lecherous thoughts about her, but he tries to block these thoughts, believing that he must protect her from everyone, including himself, and tries to make himself think of her as a little sister, later on, their relationship strengthens and rather than seeing her as his little sister, Ise starts seeing her as a single girl and a woman, this happened due to the fact that both of them kissed each other at the end of Volume 6 in the light novels. She often gets teary-eyed when other girls flirt with him. When Issei died, she, along with the others, lost their will to fight, but regained hope after hearing that his spirit was still alive.
When Akeno first met Issei, she thought of him as a kouhai that she could spoil and flirt with, but as time progressed, she started to see that Issei was very protective and started to fall in love with him. She truly fell in love with him in Vol. 7 after he helped her release her Holy Lightning ability and reconciled her relationship with her father. She loves him a great deal and isn't afraid to get into fights over him with the other girls as she does get jealous, though she is a lot more calm about it than Rias and the others. She is fine with the belief that she is third in his harem after Rias and Asia, as she likes the idea of an affair. Issei does care a great deal about her, and is willing to put his life on the line for her. He also has lecherous thoughts about her almost as much as Rias. She was heartbroken and cried terribly with her father when she heard that Issei died, and was terribly depressed by the news. She found out later that Issei's spirit is alive and her power was upgraded by Azazel's bracelet, giving her access to her Fallen Angel form in which her Holy-powers drastically increase.
At first, Koneko hated Issei due to his lecherous mind and antics, but started to like him after seeing how he never gives up and is willing to do anything for anyone. She started to have feelings for him in Volume 5 after he helped her overcome her fear of Senjutsu because Issei told her he would stop her if she went out of control. After that, she began sitting nowhere else but on Issei's lap, and began seducing him like all the other women in the harem. She often gets jealous like the others if another girl is with him or hugging him.
Her love kept growing and growing until it reached its climax in Volume 11 where she went into mating season and chose him as her partner, but was stopped by Kuroka. Later in the same volume, she proposed to Issei to be his bride, which Issei accepts under the condition that she grows bigger. She wants to be more than just a girlfriend to Issei because she chose him to be her soulmate. Her relationship with him is very serious, but will have to wait to develop more because her body is small and would be a danger to her to bear children at this point. When Issei died, she cried along with Ravel and was also happy to hear his spirit was alive.
Kiba is the Knight of the Gremory group and is the self-proclaimed best friend of Issei. Issei hated Kiba at first due to him getting all the girls' attention with his good looks, but Issei started to like Kiba as a good friend. They both want to become strong so they can protect the girls of the Gremory group. Issei and Kiba have a good friendship and are willing to protect each other from harm. When Issei died, Kiba felt sadness and anger but kept it in. When he found out Issei's spirit was alive, he released his anger on Siegfried and gained hope when Issei's Evil Piece turned into Ascalon for him to use. He deserves five fiance, just like Issei.
Irina and Issei have been friends since childhood and reunited in Vol. 3, although Issei was surprised to see her because as kids he thought she was a boy. Issei likes her very much and has lecherous thoughts about her like the other girls. She has had a crush on Issei since they were little but was disappointed to see that he had become a Devil and a pervert but nevertheless still loved him. She wants to make love to Issei and have his children like Xenovia to apparently strengthen Heaven's forces. She loves Issei very much but is having trouble in the harem because if she thinks or does anything bad, she starts to "fall". However, it is suggested she is willing to fall for Issei's sake. Issei wants to give her babies as well and wants to protect her just like the other girls.
Xenovia and Issei met in Vol. 3 and did not care much for Issei. Issei initially disliked her due to her calling Asia a witch and wanting to kill her, but after Vol. 3, she apologizes greatly and becomes a Devil after learning of God's death. She then decides that she wants to have strong children and chooses Issei to be her mate. She is completely oblivious to sexual situations due to her being raised in the Church, and tries to have sex anywhere with Issei, even going as far as to bring condoms to school so they could "practice".
Later on, she develops strong feelings for him because Issei protects her quite a bit, which she is fond of and therefore falls in love with him. She does not get jealous when girls are around him, often at times just letting it go, although when one of the girls tries to have sex with him, she becomes jealous and tries to take him away. Issei is willing to protect her just as much as the other girls, and loves the fact of having babies with her.
When Gasper and Issei first met, Issei thought that he was another girl and was heartbroken after learning that he was a cross-dressing boy. Issei often cursed him for being a shut-in, although Issei and Gasper become great friends and Issei trains him as his junior. Issei's blood gives Gasper a boost in power, even making him able to hit his Sacred Gear's Balance Breaker for a short time. When Issei apparently died in Volume 11, he seemed to transform, becoming filled with rage at the Khaos Brigade for killing his friend, and proceeded to defeat Georg with his new power.
Rossweisse is the second Rook of Rias' peerage and a teacher at their school. She never had a boyfriend, and was constantly teased by Odin about it. At first she had no interest in Issei and found his predicament with his harem funny and often teased him. In Vol. 12, she starts to come around Issei, seeing that he is a caring person and shows interest in him to the point of wanting Issei to have multiple dates with her because she likes him.
Ravel and Issei met during the Rating Game between Rias and her brother Riser and didn't really care for each other. After he defeated Riser at the engagement party, she became infatuated with Issei and in Vol. 5, she starts to develop feelings for issei, albeit in a tsundere fashion. In Vol. 10, she moves in with him and transfers to his school to spend more time with him. She stated that she does not mind becoming one of his concubines as long as she is by his side, which shows that she truly loves him despite being a tsundere. Issei, however, remains oblivious to her feelings but was still willing to put his life on the line to protect her because Ravel's mother told him to take care of her.
Sona at first treated Issei like a regular servant, but as time went on she admired him for his tenacity and courage to protect Rias and the other servants. She respects him greatly for everything he has done for Rias but she is jealous as well because Issei solved many of Rias' problems for her and she, as her best friend, could not do anything about it due to her position as a High-Class Devil. She considers him a great friend and suggested to Issei to call her by her first name (which he considers to be an honor), and told him to take care of Rias even though she's stubborn and has a great deal of pride. She has admiration and respect for him for his accomplishments and everything he has done for Rias, and she also congratulated him on his promotion recommendation to Middle-Class Devil.
Saji and Issei have a friendly rivalry going on because they are both Pawns, they both harbor feelings for their masters, and they both have Dragons sealed inside their Sacred Gears. At first they did not care for one another, but as time progresses they eventually became good friends , even stating that they are "War Buddies" in Volume 3 of the light novels,and have started training to become strong to face one another again. When Issei died, Saji showed expressions of anger and sadness due to him being killed and wanted to avenge Issei.
She congratulated Issei for his promotion recommendation to Middle-Class Devil and asked for his autograph, saying that Issei is the type of guy that she likes because he is always muddy after a fight, and admired Issei for his fight with Sairaorg Bael.
Raynare was Issei's first girlfriend, in which she disguised herself as Yuma Amano to trick and deceive Issei in order to kill him because he was deemed a "threat". She killed Issei with a light spear at the end of their date, and afterwards killed Asia by stealing her Sacred Gear. This caused Issei to hate Fallen Angels altogether. Issei later defeats Raynare and was almost tricked back due to her becoming Yuma once more, trying to convince him to spare her life, but he sees through her ruse and allows Rias to kill her.
Although dead, Raynare left a permanent mark in Issei's heart, as explained in Vol. 10 when Koneko, Asia, and Akeno try to "heal" Issei. She made Issei fear women after he planned his whole first date with Raynare, only to be killed and treated terribly by her. She also made him afraid to call girls by their first name (i.e. calling Rias "President (Buchou)" instead of her name), and also made him afraid of being disliked by other girls or rejected if he showed his feelings again. The girls, however, quickly object to this, and Raynare's "curse" is lifted from Issei's heart.
Issei, at first, hated Azazel because he was a Fallen Angel, but learned later on that he means well and has good intentions. Issei respects Azazel a lot, even calling him as "Sensei" after Azazel tells him that he would teach Issei the secrets of the Sacred Gear and how to ensnare beautiful women. Azazel later became the Occult Research Club's advisor in Volume 4, and aided the Gremory team on how to better improve themselves as a team. He also helped out in progressing Issei and Kiba's Sacred Gears, as well as help Issei learn how to achieve Balance Breaker and control the Trianna. He also shows a great deal of concern for Issei after his death but is relieved to hear his spirit is alive.
Baraqiel at first hated Issei because he misunderstood the meaning of Oppai Dragon as a Dragon who feeds off of breasts, instead of a Dragon who loves them, and as such believed that Issei wanted to eat the breasts of his estranged daughter, Akeno. However, since coming to understand the mistake and having been reconciled with his daughter because of Issei, Baraqiel has his shown approval and gained respect for him.
Issei sees Michael as a very powerful leader and respects him very much. Likewise, Michael has also shown a great liking towards Issei due to his kindness and power. In Volume 4, Michael gave Issei Ascalon as a symbol of peace between the Three Factions prior to the events of the Kuoh Treaty.
Sirzechs is Rias' older brother and met Issei during his duel against Riser at Riser's engagement party. He shows a great interest in Issei and considers him part of his family, even asking Issei to call him his big brother. He showed anger at the death of Issei and, like the others, wanted to avenge him. He was also relieved to hear his spirit was still alive. He has shown great faith in Issei's relationship with Rias as they progress on. He passed his role to take pictures of Rias sleeping to Issei, though Rias was upset and said she didn't mind Issei doing it but heavily rejected Sirzechs with doing such things.
She has a minor rivalry between Oppai Dragon and her own magical girl television series, but otherwise appears to like Issei.
Appears to like Issei, even being willing to illegally modify his Evil Pieces so as to allow him to promote at any time. Whether this is more of an experiement on his part, a favor to Sirzechs, or for some other reason, is unknown.
Lord Gremory is Rias' father and meets Issei through her (although after the fight with Riser). He at first was disappointed at the fact Rias did not marry Riser to connect the families, but later accepted his daughter's content of loving Issei. He soon became fond of Issei and becomes good friends with Issei's parents and accepted him happily into his family, even wanting Issei to call him father. Like Sirzechs, he also shows great faith in Rias and Issei's relationship, although he often embarasses the two (more often Rias). He also supports Issei's harem.
Venelana is the mother of Rias. She, like her husband, was also very disappointed at the fact the the engagement between Riser and Rias broke off. She, however, became fond of Issei, with Issei having some lecherous thoughts of her because of how young she looks and how she bears a striking resemblance to Rias. She teaches Issei how to dance and eat fancily and become a proper gentleman so that he does not embarrass Rias at social events in the Underworld. She accepts her daughter's decision to love Issei and comes to like Issei and accepts him into the family. She supports the idea of the harem and tells Issei to always protect the girls and keep them happy.
Grayfia is the wife of Sirzechs and Rias' sister-in-law. She first met Issei while retrieving Rias, who was in the middle of trying to make love with him. She was there after the Occult Club's defeat in the Rating Game against Riser, and helped Issei bring Rias back and stop the engagement. She shows great concern for Rias but accepts Issei as her love. Like the rest of the family, she seems to be fond of Issei. In Vol. 12, she was one of the few to not even consider the possibility of his death, stating that she 'would not allow the person who will become her brother-in-law to perish with something like this [Shalba Beelzebub and Samael's curse]'. At the onsen in Izu it is revealed that she is very happy about Issei joining family and treats him as her little brother and washes his back, much to his surprise. She also demands to be called "Ane"( elder sister)- much like Sirzech.
Like his father, Millicas likes and respects Issei calling him "Issei Nii-Sama". In fact, when Sirzechs ask Millicas on whether he likes Oppai Dragon or Satan Red more, Millicas chose Oppai Dragon much to the shock of his father.
She is the mother of Ruval, Ravel, and Riser. She met Issei in Vol. 10, telling him to take care of Ravel, knowing that she is in love with him and was willing to be one of his concubines. She shows faith in Issei to please all of the girls and protect them.
Riser was the main antagonist in Vol. 2. Issei hated this man with a passion at first because he was "stealing" Rias away from the group, which Issei did not want. At the end of the Rating Game between Riser and Rias' peerages to determine whether Rias should wed Riser, Issei went against Riser but ran out of stamina and was beaten to near-death before being saved by Rias, who surrendered in order to prevent any further harm to him. He woke up two days later and went after her, interrupting the engagement party, and Sirzechs decided on a one-on-one battle. During that fight, Issei gave up his left arm to reach his Balance Breaker and ended up defeating Riser and taking Rias. Issei made him go into a depression due to the fact that he has never lost and also made him afraid of Dragons. Later on during the invasion of the Underworld, he overcomes his fear of Dragons due to Tannin's training and went out fighting and also tried to encourage Ravel about Issei's death. Issei didn't really hate him after he took back Rias. Riser still is jealous of Issei's relationship with Rias, and wants to fight him again for her.
One of Riser's Rooks. After Issei defeated her in the Rating Game between their masters, Isabela started to respect Issei's strength.
Sairaorg is the cousin of Rias who met Issei when they were deciding Rating Game matches between the young Devils. He is known as the strongest young Devil. Issei and him got off to a great start, wanting to fight each other from the beginning. They then sparred before their match, in which Issei found him to be extremely powerful and Sairaorg finding Issei to be interesting.
During the Rating Game match between Rias and Sairaorg's peerages, Issei and Sairaorg went head-to-head in a fist-fight match. Later, Issei found that Regulus, his Pawn, was a Sacred Gear which he formed with and Balance-Breaked with. The fight continued and ended with Sairaorg being knocked out while still standing. After the match, the two became good friends and promised to get stronger to fight each other once again later on. When Issei apparently died, he came and encouraged Rias to get up from her depression. He believed that he was not dead due to the fact that he had not had sex with the women he loves.
Vali is the holder of the Divine Dividing Longinus and Issei's rival. When they first met, Vali was on a another level compared to Issei, which made Issei scared but devoted to become more powerful. During the signing of the peace treaty between the three factions, he went against everyone and formed a team in the Khaos Brigade. This made Issei hate him and after which they fought with Issei in Balance Breaker mode due to Azazel's bracelet. Issei won the fight but Vali was teleported away. Later on, however, the two joined forces to defeat Loki, and after that, began cooperating. Even later into the series, the two started to become friends, although still in a rivalry. They promised to fight each other someday and to become even stronger.
Kuroka is the elder sister of Koneko. She met Issei in Volume 5 when she tried to take Koneko, with Issei and Rias stopping her. He fought her for Koneko and almost punched her in the face but stopped at the last moment, warning her that if she ever tried anything again, his fist wouldn't stop. Later on, Issei found that she loves her sister very much and found her to be very sexually appealing. During Koneko's mating season, she made matters a little worse by stimulating Koneko and Issei. Although she stopped her sister, she still began to seduce Issei because she wants his "genes" for strong babies, which she tried with Vali but he denied. Koneko vehemently protests by protecting Issei, saying that Issei is hers. In Vol. 11, Issei tries to protect Koneko but is unable to do so because of the injuries sustained during his fight with Cao Cao. However, Kuroka protects her, after which he learns about her past while she is healing and tells Koneko she is a good person, although a "naughty cat".
Le Fay is the witch of the Vali Team whom Issei meets in Volume 9, where it is revealed that she is a huge fan of his show, Breast Dragon Emperor Oppai Dragon (Oppai Dragon for short), wanting to shake hands with Issei and asking for an autograph upon meeting.
Ddraig is the Dragon known as the Red Dragon Emperor that is inside Issei's Sacred Gear/Longinus, Boosted Gear, and has the ability to double his power as well as other abilities. He seems to hate Albion, the White Dragon Emperor, though the reason for it is unknown. He gets along with Issei very well, but later on when everyone starts calling him the Oppai Dragon, he becomes extremely depressed due to his credit being demolished little by little. Going so far as to require medicine and counseling for the depression. He has a great interest in Issei due to him never giving up and protecting the ones he loves. When he was with Issei when he died, he was devastated and told Ophis he was the greatest possesor of him, although he comes back along with Issei.
Tannin is the Dragon Issei trained with in the Underworld and helped him become strong and attain his Balance Breaker. Issei and Tannin have such a close relationship that Issei calls him "Old-man Tannin". The two are really good friends and have fought numerous battles with each other. He was, at one time, a Dragon King and knew Ddraig before he was sealed. When Issei died, he was sad along with many others but rejoiced when he came back.
Ophis officially met Issei at his house when Azazel invited them over to negotiate. At first, he was afraid of her because he thought she would destroy them immediately. Although she has no intentions other than to defeat the Great Red and return to her birthplace in the Dimensional Gap. At first, she took a huge interest in Issei because of how Ddraig was and reacted with Issei and how his color had changed from red to crimson. In Volume 11, Issei became Ophis' first friend, and he invited her to live with him in the Hyoudou Residence. Ophis is also Issei's last friend (according to Ddraig) prior to his reincarnation as a humanoid Dragon (later Dragon/Devil hybrid). In Vol. 12, she plays a big role in reviving Issei with the Great Red's help, and now stays with Issei.
The relationship between Issei and the Great Red is somewhat vague, as the Great Red rarely communicates wniggers diedith anyone and most of the communication between the two has been through Ddraig. However, the Great Red was willing to save Issei and Ophis when the pocket dimension they were in collapsed, hurling them into the Dimensional Gap. He also crafted a new body for Issei from his own flesh. This seems to indicate that the Great Red likes Issei, possibly because of Issei's powerful dream to become Harem King and the Ultimate Pawn.
Because Issei's present body has been made from the Great Red's, Issei can merge with his power with no effort.
Issei thinks of the Great Red as almost like a super-powerful delinquent because of his tendency to just fly around the Dimensional Gap and taking offense at being glared at. Issei is also embarrassed by the Great Red's apparent fondness of the Oppai Dragon theme song, a line of which is the only words directly exchanged between the two.
Despite the fact that Issei constantly complains about his parents, Issei does love them deeply. In Volume 4 when Vali mentioned that he should try killing Issei's parents, Issei got so angry that he wanted to kill Vali on the spot.
They are Issei's two closest human friends and fellow members of the Perverted Trio. The three will often get together and perform such activities as peeping and watching/reading porn. However, when push comes to shove, the two are completely willing to leave Issei to his own devices and receive their share of the punishment. The two have become very jealous of Issei's newfound popularity with the ladies, but still hang out with him to indulge in their hobbies.
Issei always complains about Kiriyuu that she always teaches/tells Asia erotic things.
She met Issei when Rias needed to write a report about youkai in Volume 8, but she didn't want to do it because she fears she might lose her soul should she get involved with Devils. However, she would do it on the condition of whoever wins against her in a tennis match, with the conditions being if Rias lost, she would have to give up Issei temporarily. Unfortunately for her, she lost and had to write the report. Later on, she asked Issei to help her break an engagement that her father set up. Issei decided to help her in a tennis match against her father, and in the process of helping her, she fell in love with him.
She has a crush on Issei after what Issei did to help in Kyoto and she is the Leader of the Youkai in Kyoto.