Issei Hyoudou is the main protagonist of the series. He is a second-year student at Kuoh Academy, where he is known as an extremely perverted individual with a bad habit of fantasizing about his female peers. Issei is also surprisingly hot-blooded and hard-working. His first girlfriend suddenly kills him, but Rias saves him and brings him back to life as a Devil of her own clan, thus making him a member of the Occult Research Club. His rank is "Pawn",(he later became "King") and while his powers as a Devil are low-ranking, he wields a Sacred Gear on his left arm called "Boosted Gear". He "works" hard day and night for the sake of his master and the highest-ranking Devil, Rias Gremory. Issei's primary goal is to one day become a high-ranking Devil so that he can create his own harem and become a "Harem King". In reference to his personal appearance in the anime and the light novels, it has been revealed that he is "cute" rather than "good looking". However, later in the storyline, it is stated by many female members both of his peerage and outside of it that he has grown manlier and more muscular.
Regardless, Issei is still a somewhat shallow, and perverse character. He appears to care more about the physical appearance of his female peers and shows little to no regard for their feelings. He also is immature, which can be inferred by his goal of becoming a "Harem King".
In the light novels, it is revealed that Issei's Sacred Gear has more capabilities then what has been shown before. The Boosted Gear is actually a Forbidden Gear, which starts off as a gauntlet at its first stage. While training and mastering its powers, the Sacred Gear will spread across his body, serving as armor. This is because the gauntlet doubles the user's power every 10 seconds without limit, the only restriction being the user's own ability to handle the power. Hence why Angels, Fallen Angels, and Devils fear this power.
In Volume 4 of the light novels prior to the meeting of the three factions, he receives St. George's Dragon Slayer sword from the Archangel Michael. Due to his training in the Underworld with former Dragon King,Tannin, he obtained a great stamina that he can use when he gained his Balance Breaker, which he does during his fight with Kuroka where he gained full-access to his Balance Breaker. Issei has a tokusatsu show in the Underworld called Breast Dragon Emperor Oppai Dragon (乳龍帝おっぱいドラゴン Chichiryūtei Oppai Doragon?), which stars him as the main character named Issei Gremory and is popular among the children.
Issei's Sacred Gear, "Boosted Gear" (赤龍帝の籠手（ブーステッド･ギア） Būsuteddo Gia?, lit. "Red Dragon Emperor's Gauntlet"), is one of the Thirteen Longinus, where one of the Two Heavenly Dragons, Ddraig, the Red Dragon Emperor, resides. Issei is nowhere close to mastering his Sacred Gear, and as a result when he enters Juggernaut Drive, 99 percent of his lifespan is sacrificed, though he and Koneko managed to get it back. During Rias' Rating Game against Riser, Issei's Boosted Gear transformed to its second form, called the Boosted Gear Gift (赤龍帝からの贈り物(ブーステッド・ギア・ギフト) Būsuteddo Gia Gifuto?, lit. "Gift of the Red Dragon Emperor") which allows him to transfer his boosted powers to other people or objects. During his fight against Vali, Issei placed one of Albion's jewels on his right gauntlet, resulting in the creation of the "Dividing Gear" (白龍皇の籠手（ディバイディング・ギア） Issei was banned from using it by Azazel, because it shortend his lifespan whever he used it. Issei can also fire a beam of energy from his Boosted Gear by concentrating his magical powers, which he dubs the "Dragon Shot" (魔力弾（ドラゴンショット） Doragon Shotto?). In Volume 12, it was revealed that after seeing Sirzechs use his Power of Destruction, Issei had been working on controling his Dragon Shot, allowing him to redirect it in midflight so that it could hit different targets as he wished. During his fights against the Hero Faction of the Chaos Brigade, Issei developed a technique to fire off a barrage of mini-missles with only enough explosive force to knock out, but not kill, a normal human. In Vol 12, Issei unlocked a forbidden power of the Balance Breaker called Longinus Smasher. A powerful energy blast fired from a cannon hidden in the chest of the Scale Mail. Issei had previously used it against a member of the Old Maou faction while in Juggernaut Mode, but had no memory of it. While merged with Great Red, the power of Longinus Smasher was enough to obliterate a large portion of the Underworld, but its full power without that boost is presently unknown.
Issei himself has also invented an array of techniques that go with his perverted nature. During the training in preparation for Rias' Rating Game against Riser, he invented the Dress Break (洋服崩壊（ドレス・ブレイク） Doresu Bureiku?, lit. "Clothes Collapse"), which allows him to strip the clothes of females after coming in contact with them. He also invented the attack "Booblingual" (乳語翻訳（パイリンガル） Bairingaru?, lit. "Breast Translation"), which allows him to talk to the breasts of females and learn his opponents' plans and strategies. However due to complaints from the female fighters, this technique was banned in Rating Games.
In Volume 8 of the novels, Ajuka Beelzebub, one of the Four Satans and the original creator of the Evil Pieces system, remodels Issei's Piece so that he can use Promotion anywhere and at any time. By using Promotion, he is able to use Knight, Rook, and Bishop ranks, and can achieve Illegal Move Trident (赤龍帝の三叉成駒（イリーガル・ムーブ・トリアイナ） Irīgaru Mūbu Toriaina?, lit. "Red Dragon Emperor's Three-Forked Promotion"), which makes Issei's Balance Breaker, the Boosted Gear Scale Mail (赤龍帝の鎧（ブーステッド・ギア・スケイルメイル） Būsuteddo Gia Sukeiru Meiru?, lit. "Armor of the Red Dragon Emperor") armor, change to other forms using Promotion.
- In Knight Form, called Welsh Sonic Boost Knight (龍星の騎士（ウェルシュ・ソニックブースト・ナイト） Werushu Sonikku Būsuto Naito?, lit. "Dragon Star Knight"), he possesses enormous speed that far surpasses Sairaorg's speed, but makes his armor thin where it has low defense.
- In Rook Form, called Welsh Dragonic Rook (龍剛の戦車（ウェルシュ・ドラゴニック・ルーク） Werushu Doragonikku Rūku?, lit. "Rigid Dragon Tank"), he possesses massive power and defense but has a low speed because of its massive armor.
- In Bishop Form, called Welsh Blaster Bishop (龍牙の僧侶（ウェルシュ・ブラスター・ビショップ） Werushu Burasutā Bishoppu?, lit. "Dragon Fang Monk"), he possesses an enormous amount of magical energy in which he applies it to the two cannons at the back of his armor, but takes time to recharge.
- In Volume 10 of the light novels, he later gains access to a Queen Mode, called Cardinal Crimson Promotion (真紅の赫龍帝（カーディナル・クリムゾン・プロモーション） Kādinaru Kurimuzon Puromōshon?, lit. "True Crimson Dragon Emperor") which is equally as powerful as Juggernaut Drive. Juggernaut Drive is the full-bodied armor of the Boosted Gear, the Scale Mail, which gives him the Balance Breaker ability.
- Super Gigantic Balance Breaker (超巨大禁手?) is a form Issei attained in Volume 12 by gaining Great Red's help, in which Issei reproduces the size of the Great Red that he uses to destroy the beast created by the Annihilation Maker. In this form, he gains highly destructive powers that can cause an area in the Underworld to disappear without a trace.
At the end of Volume 10 of the light novels, Issei cannot hold back his feelings for Rias and confesses to her, calling her by her first name for the first time. She accepts his feelings in tears of joy and the two officially become a couple, although it seems none of the other girls (Akeno, Xenovia, Asia, Koneko, Irina, and Ravel) have given up on Issei. In Volume 11 of the light novels, Issei took a promotion test along with Akeno and Yuuto to be promoted from low-ranked Devils to middle-ranked Devils, and Rias' mother suggests that Issei and Rias get married. In Volume 12, Issei's original body is destroyed due to Samael's curse but gains another body through the help of the Great Red and Ophis. At the end of Volume 12, Issei is officially promoted into a Middle-Class Devil. Due to the effects of having a body of Great Red's flesh he can at will turn any part of his body into a dragon.
At the end of Volume 20 of the light novels, Issei gains the power to use Cardinal Crimson Promotion [Diabolos Dragon] with powers equal to that of a super devil through the help of Ophis. Issei can only achieve this because the foundation of his body came from Great Red's flesh, allowing him to withstand Ophis's power. In this form, he is able to defeat all four great satans. Making him the strongest being in the universe.
- Issei birthday is on April 4.