|“||Yeah! I'll show him! I'm more than just the Red Dragon Emperor...I'm the Crimson Dragon Emperor!||”|
–Issei, Episode 4: Days of Judgement (Part I)
In the previous timeline prior to his death, Issei was a member of the Occult Research Club, and his rank in Rias' peerage is Pawn. His power as a Devil ranks at the bottom in the beginning (which increases as the series progresses, until eventually he becomes the strongest among the young Devils and develops his strength into SS-Class); however, on his left arm, he wields the Sacred Gear, Boosted Gear, making him a dangerous opponent to underestimate. His initial goal was to become a High-Class Devil and amass his own peerage consisting of women, in order to fulfill his dream of becoming a "Harem King," though after the events of the previous timeline, he feels as if he has already accomplished such a goal, having all the girls, sans Rossweiss, who was not a member at the time, in the Occult Research Club as his lovers.
In the current timeline, he is regarded as the Crimson Dragon Emperor by many individuals, and one of the warriors that defy the Law of the Universe.
Issei is a young teen with brown hair and light brown eyes, standing at '5, 7" ft tall, and weighs 62 kg.
While he has worn various outfits throughout the series, his most commonly worn outfit is the Kuoh Academy boys' school uniform, which consists of a blazer (more commonly black, although in other media is shown with a tinge of purple or grey, with white accents) over a white, long-sleeved dress shirt with black highlights with a black ribbon on the collar, matching black pants, and brown dress shoes. However, Issei's uniform differs in the fact that he wears a red T-shirt underneath his open dress shirt and blazer, and wears blue and white sneakers in place of dress shoes.
Issei has many personality traits. Issei is first shown to be ignorant, self-centered, arrogant, and his ongoing perverted nature, often to make lecherous fantasies, and utterly obsessed with women's breasts and having his own harem. This has caused him to be instantly labeled as a pervert among the girls of Kuoh Academy and has gained him notoriety for being one of the members of Kuoh Academy's Perverted Trio.
Despite his pervertedness, Issei is kind, courageous, and unwaveringly determined of wanting to protect those who are important to him despite almost dying by the hands of his opponents. He's also very caring, "putting his nose on other peoples business" as he helped Yuuto, Koneko, and Akeno overcome their fear and hatred of their mournful past. Due to this, he gained strong friendship from his comrades and affection from most girls of the Occult Research Club. He is, however, oblivious to the feelings of the girls around him (save for Rias after his confession), often believing they are simply teasing him. Even with Rias' family telling him to use the same titles for them as Rias (Father, Mother, Brother, etc.) and other mentions of engagement activities, he still remains the same. This is due to his trauma with Raynare.
After his murder by Raynare, Issei had developed a fear of admitting his feelings to the girls, as he thought it would cause them to turn their backs on him. He acknowledged that this would never happen, but it was still an intense fear. However, by Volume 10, that fear had been overcome. Even with knowledge that the girls do have romantic affection for him, he still has problems acknowledging some of their ongoing rivalries, an example being who is the one to sit on his lap. In battle, Issei is sometimes very stubborn, and due to his emotions, he often charges ahead towards his enemies without a second thought whenever his comrades are insulted, treated badly, or easily defeated. Azazel has stated that it is one of Issei's bad habits. Another sign of his kind nature is that despite wanting to lose his virginity, Issei refuses to take advantage of women when they are upset, even if they themselves are willing to. This happened when Akeno appeared in front of him naked and wanted to have sex to forget about her sad past and her father. However, Issei refused, knowing it will just hurt her more, and put her clothes back on and hugged her to comfort her.
In the current timeline after the appearance of the Triangulum, Issei has matured greatly, fully aware of the girls' feelings for him, and has become slightly more protective of them, having seen their corpses moments before his death. He also take things more seriously, and learns how to adapt in battle, finding new ways to defeat his opponents. He has also started showing signs of becoming influenced by Boosted Gear, as his predecessors have, as he wishes to become the True Crimson Dragon God Emperor, and defeat Great Red, therefore marking himself as the strongest existence in the Universe.
Initially, Issei's parents had a great deal of trouble conceiving a child, failing twice. So when Issei's mother got pregnant for a third time his father had paced back and forth in front of shinto shrine honestly wishing that this time they would succeed. When the child was finally born he was named Issei, meaning "Honestly," for his father's honest prayers.
He is a childhood friend of Irina Shidou and they would always play together before she moved to England, although he thought she was a boy at the time. At some point when Issei came over to Irina's house, she kissed him while he was sleeping. Sometime during his childhood, Issei met an old man who told him and other children stories about breasts, which eventually led to Issei's fascination with them, and his perverted tendencies.
He attended the same middle school with Matsuda and Motohama and the three of them started attending Kuoh Academy one year before the story, and have come to be known as the Perverted Trio to just about the entire school.
During his second year, Issei met a young girl named Yuuma Amano, who asked him if he would be her boyfriend. This incident would eventually lead to his death, and later revived as a Devil, and under the service of a noble by the name of Rias Gremory. From there on, Issei would be thrust into many events, adjusting to his new life as a Devil, and unknowingly finding himself in the middle of a harem, which had been his lifelong dream. However, at some point in time, shortly after the Young Devils Gathering, Issei encountered a mysterious man who referred to himself as the Anguished One, and later found himself in a battle against a group of beings known as the Triangulum. Details regarding this point in time are scarce, but what is known is that all of Issei's comrades perished, including himself.
Not all was lost, however, due to the intervention of the Anguished One and Vali, as they reversed the flow of time. As a result, Issei is sent back to the past, six months before his revival as a Devil.
- Issei's birthday is on April 16.
- Issei's appearance is based off of Kazuma Torisuna from s-CRY-ed because the author is a fan of the series.
- Issei is a fan of Dragon Ball (Drag-so Ball in the Anime and Manga).
- In the Light Novel, Issei's Dragon arm is depicted as a scaly Dragon arm that covers his entire left arm, while in the anime, his Dragon arm is replaced by the Boosted Gear.
- The manga depicts Issei's Dragon arm as a combination of both.
- Issei had a tokusatsu show in the Underworld in the previous timeline called Breast Dragon Emperor Oppai Dragon (, Chichiryūtei Oppai Doragon), which stars him as the main character named Issei Gremory and it is considerably popular among the children.
- Four of Issei's eight Pawn pieces have become Mutation Pieces, to represent the powers of Illegal Move Triaina and Cardinal Crimson Promotion.
- In the short story "25(Twenty Five) at the Clubroom with Rias-buchou!", Issei was stated to have black hair; however, the anime and light novel illustrations portray him as having brown hair.
- According to the Magicians, Issei's strength is ranked in SS-Class in the previous timeline. In the current timeline, his strength is C-Class.
- In the 7th Chapter of The Crimson Dragon, Issei's strength was ranked as SSS~.
- His favorite food is cheesecake.