Domon Ishijima is one of the main protagonists in the narrative, known for his large, imposing physique, a prominent mohawk hairstyle, and a nose ring. He is a sixteen-year-old student at Nashikiri High School, though his academic performance is notably poor. Before the story begins, he is a feared delinquent known as Oni, which means Ogre, due to his immense strength. However, this rough exterior conceals a caring nature, especially evident in his devotion to his mother, whom he helps run her flower shop whenever he is not fighting.

Personality wise, Domon is hot-blooded, impulsive, and often serves as a source of comic relief. He tends to act before thinking and can be somewhat perverted, yet he possesses a remarkably strong will and a deep sense of loyalty to his friends. His primary driving motivation for much of the early story is his unrequited love for Fuuko Kirisawa. After being consistently defeated by her, he was told he could only challenge her again if he first defeated Recca Hanabishi. This led Domon to constantly stalk and challenge Recca, vowing to make the ninja his personal servant to impress Fuuko. Despite his many attempts, he never manages to defeat Recca, but this rivalry eventually transforms into a strong friendship. His motivations evolve beyond his own desires as he becomes fiercely protective of his allies and determined to prove himself as a valuable member of the Hokage team.

Domon's role in the story is that of the powerhouse and emotional anchor of the Hokage group. He is initially a rival but quickly becomes a steadfast ally. While he may appear to be the least skilled or intelligent of the group, his incredible physical strength and indomitable spirit often allow him to overcome enemies that his more talented teammates cannot. He is always willing to put himself in danger to protect others, particularly Fuuko and the team's healer, Yanagi Sakoshita.

His key relationships are central to his character. His bond with Fuuko is a constant source of both comedy and motivation; she frequently rejects his advances, but they share a deep mutual trust and he often fights his hardest to impress or save her. His friendship with Recca grows from rivalry into a brotherly bond, with Domon acknowledging Recca's strength while providing emotional support and comic relief. He forms a strong camaraderie with other team members like Tokiya Mikagami and later, Kaoru Koganei.

Throughout the series, Domon undergoes significant development. He starts as a simple-minded brawler focused on personal glory but matures into a reliable and strategic fighter who understands the importance of teamwork. His confidence grows as he achieves victories in the dark tournament, moving from being seen as the weakest link to a crucial fighter whose mental fortitude earns respect, even from powerful enemies like Kurei. He learns to harness not just his physical power but also his unbreakable will.

In terms of abilities, Domon's primary asset is his superhuman physical strength, which is formidable even without any tools. He later acquires several Madogu, which are magical tools that enhance his power. His first Madogu is the Dosei no Wa, or Saturn's Ring, which was secretly given to him by Kagerou and replaced his nose ring. It dramatically amplifies his physical strength, glowing brighter as his willpower rises. He later obtains the Kuchibashi-O, a massive metal beak attached to a chain that he uses to grab and strike opponents from a distance. His most notable Madogu is the Tetsugan, or Iron Ball, which he accidentally swallows. Once inside his stomach, it transforms his entire body into living steel, granting him near-impenetrable durability. However, his greatest weapon is consistently shown to be his incredible mental strength and willpower, which allows him to overcome powerful illusions and even resist attempts to erase his mind entirely.