Isamu/Synopsis

History
Sometime prior to the original storyline, Child Emperor worked as Metal Knight's assistant. He also fought and defeated all of Atomic Samurai's disciples.

Alien Conquerors Arc
Child Emperor is first seen during the S-Class heroes meeting and was slightly upset that he couldn't meet Blast, who hasn't attended. During the meeting, Sitch explained the prophecy Shibabawa made before her death, but Child Emperor didn't believe it so tried to leave. He was in return mocked by Sitch, stating that Child Emperor is nothing more than a child if he does not comprehend the danger of this matter, which angered him. After the Dark Matter Thieves attack A-City, Child Emperor suggests that the Hero Association should confirm who the enemies are and whether they are what the prophecy was referring to. He later goes to the roof of the Hero Association Headquarters with Genos, Superalloy Darkshine, Tatsumaki, and King. He immediately starts thinking about ways to take care of the spaceship, while Bang, Metal Bat, Atomic Samurai, and Puri-Puri Prisoner take care of Melzargard. While using his laptop, Child Emperor warns the Hero Association staff about the about rising energy in the ship. Afterwards, Child Emperor appears beside Genos and Drive Knight, with the latter informing his leave.

Super Fight Arc
Child Emperor uses Underdog Man No. 22 to analyze Eyesight's poisons to find a cure for the paralysis. Child Emperor then proceeds to detonate Underdog Man No. 22, but that did not successfully kill Eyesight as she hardened her skin. Eyesight then manages to find Child Emperor, but is interrupted and eaten by Pig God. Pig God decides to go after the other monsters in Y-City afterward.

Monster Association Arc
Child Emperor is investigating the tunnels left by Elder Centipede to locate the Monster Association's base. Bofoi is discussing with Child Emperor and tells him that he will not join in on the operation to raid the Monster Association. He tells Child Emperor that the Monster Association is thoroughly prepared for the Hero Association and that many heroes will be sacrificed in the process. Bofoi tells him that the Hero Association denied permission for him to plant explosives in the designated hideout of the Monster Association headquarters, because saving the hostage was their highest priority. Due to this, Bofoi will not bother with the operation. Child Emperor continues tries to convince Bofoi to partake in the raid but is rejected by Bofoi, who refuses to participate due to other potential threats. Bofoi mentions how Monster King Orochi pierced through the Metal Knight drone before it could activate its self-destruction mechanism. Bofoi suggests to Child Emperor to not trust those around him and to accumulate his power as you only need yourself to fight for justice. As a result, Child Emperor tells Sekingar to take Demon Cyborg off the roster due to a suspicious history and Silver Fang due to his past relationship with Garou.

The S-Class heroes, Child Emperor, Tatsumaki, Kamikaze, Pig God, Flashy Flash, Superalloy Darkshine, Zombieman, Puri-Puri Prisoner and Sekingar attend a meeting and they reassure Narinki that they will rescue his son. They each receive a transmitter that will show their location to each other. They discuss the situation until Sweet Mask bursts into the meeting. Child Emperor tries to calm him down and get him to accept a leadership role with the surface strike team but he refuses. Child Emperor soon leaves to join the rest of the heroes preparing to leave. He notices King is missing and wonders where he is but is told by Zombieman that he can hear the King Engine. King returns and they all leave to attack the Monster Association.

Child Emperor arrives at the ghost town of Z-City along with the other heroes. Child Emperor notices King missing and is informed of his disappearance. Child Emperor is disappointed at King's perceived lack of trust in him and they are soon assaulted by waves of monsters from the Monster Association. Child Emperor detects over 150 monsters and is steadily rising and overseer the support heroes. While the heroes win the battle, Child Emperor thinks it's odd that they send their weaker monsters out. He reasons out that they're being watched and destroys one of Gyoro Gyoro's spy drone.

After the support heroes recover, the time for the S-Class infiltration comes to a close and they take their position. Child Emperor monitors everyone's transmitters, noticing that the one moving at high speed is Flashy Flash though it has stopped moving and wonders if it malfunctioned. As Child Emperor continues to think about the situation, he is ambushed by a monster only for it to be taken out by one of his devices, Mini Octotank No. 8. He comes across the deconstructed Metal Knight unit and scans it for any data on the hostage. He finds a route directly to the hostage room and is frustrated at Bofoi's lack of cooperation. He also worries about technology being used by the monsters due to the attempted reconstruction of the Metal Knight unit.

Child Emperor comes across Phoenix Man who had destroyed his drone. After Child Emperor deploys his transparent shield he shreds Phoenix Man. He heads towards Waganma's cell and ambushes the guards there with poison gas. Sludge Jellyfish confronts them only to be quickly taken out by an explosion. They escape further ahead only to find G5 who resisted Child Emperor's initial attempts at destroying the monster.

Child Emperor deploys three of his Underdog Man to combat G5. With some assistance from Child Emperor birdlime missiles, they distract the monster long enough to escape. They come across Phoenix Man's shredded body and Waganma mentions that he recognizes the costume from a comedy show called "Animal Kingdom". The character had an ability to revive itself no matter how it dies. As Child Emperor ponders on the information, Phoenix Man suddenly arises into a new, stronger form. Child Emperor confronts the monster but is forced to summon his ultimate weapon Brave Giant.

Child Emperor and Phoenix Man's battle carries them to the nearest locator signal, the underground waterfall where Gale Wind and Hellfire Flame died. Phoenix Man uses his newfound powers to revive the corpses of the monsters within the pool of water. The Subterranean People, Gale Wind and Hellfire Flame are brought back to life as undead creatures to battle Child Emperor. While Child Emperor's Brave Giant electrocutes the Subterranean People to death, the ninjas are brought back fully to life. After an all-out energy attack, Brave Giant shuts down with Phoenix Man surviving the shot. As a last gambit, Child Emperor tricks Phoenix Man into ripping off his own costume with Tickle Tickle Bug No. 1 which he snuck into his costume when the monster attacked.

After defeating Phoenix Man, Child Emperor confirms no other hostages other than Waganma and head out to escape. Child Emperor informs the other heroes on the Locator of his success in retrieving the hostage and that they should continue with the mission as planned. Child Emperor is soon contacted by Kamikaze of a second child hostage. The hero interrogates Waganma on whether he knew of the second hostage and after receiving confirmation, heads back into the Monster Association HQ.

Original Webcomic
Upon the request from Sitch, he tracks down the Monster Association's base in Z City and mentions they could be holding a child hostage. He accompanies the other S-Class heroes against the Monster Association. He plans out how to defeat the Monster Association, particularly Black Sperm with King after Tatsumaki is defeated. Child Emperor gets punched in the gut by Garou.

Neo Heroes Introduction Arc
By the time the Hero Association is about to fall soon enough, Child Emperor becomes disillusioned with the Hero Association over the fact that two heroes he held in high regard, Metal Knight and Blast, did not come to aid them in the battle against Garou. This fills him up with resolve to become stronger without anyone's assistance. He later quits the Hero Association and joins the Neo Heroes. While visiting their headquarters, he comes across Metal Bat and Superalloy Darkshine, who have joined the Neo Heroes as well.