Dōtei, whose given name is Isamu, is the professional hero known as Child Emperor. He holds the rank of S-Class, position 5, within the Hero Association and is noted for being the youngest hero in the organization at ten years old. A resident of Y-City, he operates from his own personal laboratory.

In terms of background, Isamu is a prodigy whose genius intellect sets him apart. He was once an assistant to Dr. Bofoi, the S-Class hero known as Metal Knight. His decision to become a hero is rooted in his admiration for the top-ranking S-Class hero, Blast. Despite his brilliance, he does not neglect his education and is known to teach at a cram school.

Personality-wise, Dōtei is characterized by his analytical and calm demeanor. He is capable of observing situations and formulating plans with a level head, often acting as a strategist for the Hero Association. Although highly confident in his abilities, he is not typically arrogant, though he can become easily annoyed when treated as a mere child or when his ideas are dismissed. He possesses a strong sense of responsibility and justice, and he shows a caring side, such as when he prioritized creating an antidote for poisoned fellow heroes. A notable quirk is his habit of consuming lollipops, as he believes the sugar is necessary fuel for his intense cognitive activity. He can become irritable with other heroes, finding many of the S-Class members difficult to work with, and he once became so frustrated by a flaw in his own plan that he broke down in tears. His worldview is challenged when he discovers the hidden corruption within the Hero Association.

His motivations are driven by a desire to genuinely uphold justice, a trait he believes sets him apart from those who are heroes for fame or money. He strongly believes in relying on his own abilities and intellect. This is reinforced after the battle with the Monster Association, where he becomes deeply disappointed by the inability of other heroes to cooperate effectively, leading him to conclude that he can rely on only himself.

Dōtei plays a crucial role in the story as a tactician and key fighter during major arcs. He serves as the de facto leader and strategist for the raid on the Monster Association's headquarters. His most notable solo battle is against the monster Phoenix Man. After destroying Phoenix Man’s original body, the monster resurrects into a more powerful form. Dōtei is forced to use his ultimate weapon, a mech suit called Brave Giant, to defeat him. In a key moment, he is shown to be ruthless in his justice, killing the monster even after it reverts to its human form to prevent its return.

Key relationships include his former mentor, Metal Knight, whom he grows to mistrust due to the scientist's secretive and self-serving actions. He shares a respectful bond with the hero Zombieman, who becomes one of the few people Dōtei truly trusts and confides in. His interactions with Saitama are initially colored by mistrust, as an unfinished invention of his incorrectly marks Saitama as a weakling.

Dōtei experiences significant development following the main events of the Monster Association arc. Disillusioned by the Hero Association's internal scandals and cover-ups, he loses faith in the organization. Adding to his personal turmoil, he discovers the unfortunate meaning of his hero name "Dōtei," which is a homophone for the Japanese word for "virgin" or "male virgin". Upon realizing his classmates had been teasing him about this, he reinvents himself. He leaves the Hero Association, changes his name to Diabolical Emperor (Hidōtei), a pun on the term for "non-virgin," and joins a rival organization called the Neo Heroes, adopting a more serious and rebellious appearance.

Notable abilities center on his genius intellect, which is considered his greatest weapon. He is a master inventor and scientist, capable of creating advanced technology and analyzing complex problems, such as synthesizing an antidote from a poison sample in minutes. Despite his age, his physical capabilities are far above average; he is stronger than a bear and possesses reflexes that allow him to block attacks that A-Class heroes cannot. His primary equipment is his seemingly ordinary black backpack, which houses spider-like mechanical legs for mobility and combat, along with a vast array of hidden tools like chainsaws, boxing gloves, and energy weapons. He also has access to various support robots, such as the Underdog Man series. His ultimate creation is the Brave Giant, a powerful giant mech suit that can assemble from parts launched by a missile. However, this mech has a strict operational time limit of just over two and a half minutes. He is also a capable hand-to-hand combatant, with a technique named Soccer Dribble, where he kicks an opponent's head multiple times.