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 attending the S-Class heroes meeting and was slightly upset that he couldn't meet Blast, who was absent. During the meeting, Sitch explained the prophecy Shibabawa foretold 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 levels 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 units 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.

After putting Waganma in a shield, Child Emperor has Brave Giant generate an energy sword and shield, but before he can fight, Phoenix Man transports both of them into his Phoenix Space. Phoenix Man explains to Child Emperor that he intends to scout him to become a costume monster, stating that of all the heroes, he has the most darkness in him. Child Emperor denies this, saying that the other heroes look way more scary than he does, but Phoenix Man replies that Child Emperor appears to be the most stressed due to his inner desire for a trustworthy companion and an organization that makes the most of him. Phoenix Man offers to create a powerful organization with Child Emperor, one that can surpass disaster level God. He begins to hypnotize Child Emperor, telling him to reincarnate as the monster Brave Giant Man, but Child Emperor breaks out of his hypnosis and returns to the real world, punching him to the ground with Brave Giant.

Phoenix Man transforms into his Prominence Hawk Mode and knocks Brave Giant back with a barrage of strikes, remarking on Child Emperor's stubbornness and stating that he will change his mind once he sees the resurrected Phoenix Man's powers. He then transforms into Phoenix Fire Falcon Mode and attempts to critically damage Brave Giant, but Child Emperor retaliates by using his move 20000 Fold Rope Skip Grinder, knocking Phoenix Man into a pillar. Child Emperor uses Brave Giant to chase Phoenix Man through the Monster Association base, but Phoenix Man suddenly turns around and uses his Phoenix Flares to burn Child Emperor's hands. Phoenix Man then attempts another attack, but promptly flies into another one of Child Emperor's electromagnetic shields which he attached while flying. Phoenix Man asks Child Emperor to reconsider the offer, but the latter is undeterred and drives his fist into the powerful bird monster.

Suddenly, Phoenix Man transforms into Full Power 10000ºC Prominence, burning Child Emperor's hand again. Child Emperor desprately searches for a way out and finds a water source 50m below his current location. He releases Brave Giant's output limiter, breaking the floor and sending himself and Phoenix Man into freefall. As they fall, Child Emperor recalls Phoenix Man telling him his strength was needed, asking himself if he'd ever heard those words before.

Child Emperor and Phoenix Man reach the underground waterfall, causing structural damage to part of the Monster Assocation base. Seeing that Brave Giant's time limit is running out, Child Emperor decides to end the fight quickly, but sees Saitama in the water off to his side. The two of them attempt in vain to communicate, and before Child Emperor can react, Phoenix Man transforms into Phoenix Penguin Mode, leaping away before throwing a large chunk of ice at Child Emperor in his special attack Phoenix Antarctic Iceberg Attack.

Child Emperor, Phoenix Man, and Waganma land on top of the graveyard of the Subterranean People, with Saitama landing next to them. Child Emperor continues to fight Phoenix Man, but before the fight drags on, the latter transports them back to the Phoenix Space.

Phoenix Man points out that the Subterraneans killed each other, stating that their unity was weak and likening it to both the Hero and Monster Association. He then mentions the behind-the-scenes crimes of the Hero Association. Child Emperor initially brushes it off as a psychological attack, but Phoenix Man states that he's just talking as a friend, and then confides in Child Emperor about his power, stating that his Birdbrain costume granted him freedom. Phoenix Man asks if Child Emperor's heroism benefited him and requests him again to join forces. Before Child Emperor can respond, their conversation is interrupted by Saitama, who somehow found a pathway into the Phoenix Space. After a brief conversation between Saitama and Phoenix Man, all of them are transported back to the real world.

Child Emperor is contacted by Zombieman, who praises him for his spot-on analysis of the situation which proved instrumental in taking down enemy robots repurposed from Metal Knight's machinery. Zombieman thanks Child Emperor and offers to treat him to milkshakes after they defeat the Monster Assocation. Child Emperor gets slightly emotional and then updates Zombieman on his own progress, stating that he's doing fine.

Phoenix Man resurrects the Subterraneans, who climb over Brave Giant, immobilizing it. He throws a monster cell through a crack in Brave Giant's machinery, stating it's medicine for the disease known as humanity. The monster cell attempts to force its way into Child Emperor's mouth, but he spits it out, stating that being a hero comes from the inside. He instructs Saitama to take Waganma and hide before unleashing Brave Giant's power by using Gigavolt Smash, which wipes out all the revived Subterraneans at the cost of self-destructing. Phoenix Man mocks Child Emperor for this move, stating that only his eye was slightly damaged, but then he begins hysterically laughing and writhing around. Child Emperor reveals that he tossed Tickle Bug #3 into Phoenix Man's suit, stating that he'll eventually suffocate from laughing too much. Phoenix Man ultimately transforms into the tiny and extremely weak Monster Chick Man before attempting to run away. After being complimented by Saitama for his heroism, Child Emperor recognizes the bald hero as Demon Cyborg's friend, wondering what he's doing this deep in the Monster Association base.

Gale Wind and Hellfire Flame suddenly turn up, having been revived by Phoenix Man. They recognize Child Emperor and attack him in an attempt to lure out Flashy Flash, but Saitama quickly incapacitates and binds them. Child Emperor questions Saitama on his decision to venture into the Monster Assocation base, although he thinks that Saitama doesn't appear evil. The two heroes and Waganma are rescued by Underdog Man, who transports them up to the surface.

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.

Supreme Hero Arc
Child Emperor later shows up at the Neo Leader meeting. When McCoy introduces Infelsinave, Child Emperor thinks to himself that the Neo Heroes' PR moves are far superior to those of the Hero Association. When Metal Bat questions the decision to only have two security guards at the Neo Heroes HQ, Child Emperor states that it should be fine as long as there's no infighting.

Neo Heroes Showcase Arc
Later on, Child Emperor leads a squad of Neo Heroes in subduing the ongoing monster invasion. After they take care of a threat, Child Emperor tells his subordinates to take a break, but they deny his offer, stating that they're not tired. In his mind, Child Emperor admires the effectiveness of the armor, but states that he saw something similar in Metal Knight's lab. He wonders if Dr. Bofoi is playing the sides of both the Hero Association and Neo Heroes, stating that if the doctor is planning to become a dictator then the two of them may have to cross paths again.