The first appearance of the Chaos Hero is in an introductory dream sequence of the protagonist, where he is freed after being tormented by a demon. He appears once again in a second vision, ready to take on the master of Echo Building along the protagonist's party, before being seen for real in Kichijoji Mall. Here he is being beaten up by Ozawa's gang after trying to warn the people of the coming of the demons. After this, he joins the player and Law Hero in their plan to save Tokyo from destruction.
He later leaves the protagonist's party after helping in the rescue of the heroine from a public execution orchestrated by Yuriko, deciding to exact his revenge upon Ozawa by assisting the Resistance in their plan to seize his gang's base and capture him—as he made an alliance with Gotou and become a "Demon Summoner" himself.
After the events of the Great Cataclysm, the Chaos Hero appears once again before the protagonist, this time in the Diamond Realm, stating that he was teleported there "out of the blue". He then once again joins the team and accompanies the protagonist back to a post-apocalyptic Tokyo, where they discover that 30 years have passed. They meet an aged Ozawa in the ruins of Shinjuku Mall, where thanks to a deal he struck with the nearby demons, he rules with iron fist, brainwashing anyone that opposes him with the powers of the Soul Diver. The Chaos Hero ends up confronting him, but is quickly defeated by Ozawa's demon minion, Minakata. That is when he finally realizes that the fastest way of gaining strength is through demonic power, and in order to gain it, he must fuse himself with a demon. After doing so in a Cathedral of Shadows (forcing the player to sacrifice the highest level demon of Chaos alignment in his COMP), he challenges Ozawa to a rematch and easily defeats Minakata. No matter if the player decides to spare Ozawa or not, the Chaos Hero steps in and finishes his old rival himself. He permanently leaves the group after this, believing that their ideals differ greatly from his own and that they must go about their own ways, so that neither would endanger the other.
Later, he appears in Ginza, warning the protagonist not to go to Ikebukuro. If the protagonist chooses to keep going anyway, the Chaos Hero chides him for trespassing on Gaean territory and takes him into custody to be judged by Yama, the ruler of Ikebukuro. If the protagonist is Law-aligned or is Neutral-aligned and chooses not to listen to Yama's request to aid the Gaeans in the destruction of the Basilica, the Chaos Hero frees him from his cell in honor of their friendship. After witnessing the defeat of Yama at the hands of the protagonist, he realizes that he must get stronger than even Yama himself. If the protagonist is Chaos-aligned or is Neutral-aligned and agrees to help Yama, he expresses relief that the protagonist was let off and departs. Following this event, he can be found at the entrance towards the inner depths of Tokyo Destinyland, accompanied by a woman named Rie. There, he reveals that he joined the Ring of Gaea in order to stop God's machinations, and will confront the protagonist if he is Law aligned. In case he's Chaos or Neutral, he will instead advise him to meet Echidna, the master of Tokyo Destinyland. The Chaos Hero is positive that after listening to Echidna, the protagonist will finally see that the Chaos Path is the right one to take. If the protagonist takes his advice he must fight the angel Haniel instead of Echidna.
The protagonist once again encounters him, this time arguing with the Law Hero near the heart of the Basilica. He tells the protagonist about the Four Heavenly Kings, and after their defeat and the flooding of Tokyo, he requests the player aids the demon Gaean general, Ravana, in the battle against his Messian rival Vishnu—who is trying to take down Ravana's barrier that is impeding God from coming to Earth. If the protagonist chooses to stay on the path of Neutrality or Law, the Chaos Hero will state that he is tired of waiting for him to join his cause, and as such, they will fight as enemies the next time they meet. In the seventh floor basement of the Basilica, this does occur, and the Law Hero also dies by the Chaos Hero's hand if the player is Law aligned. In Neutral alignment, the Chaos Hero condemns the protagonist's desire for balance as a "waste" and claims that balance exists only to be tipped in the favor of the strong. He declares that the protagonist is as foolish as the rest of humanity and challenges him to fight, confident in his victory. This results in the Chaos Hero being killed in a battle against the protagonist. In his final moments, he laments that he is dying when he believed he possesses the power to surpass all and realizes that the protagonist always was a step ahead of him. He briefly questions whether his experience was fabricated before finally dying without regret.
If the player took the path of Chaos, the Chaos Hero finds out that Rie was Lilith, who was just using him to get near the protagonist all along. This seemingly disappoints the Chaos Hero, and he asks to be left alone for a short while. After the protagonist recovers the Devil Ring, the Chaos Hero wears it in an effort to open the door to the Seraph Michael. In his arrogance, he proclaims it gives him more strength than that of God, but his body fades away because of the power. The Law Hero then appears and gloats that the Chaos Hero brought his death upon himself for defying God's will. Shortly after the Law Hero is defeated.
He is given the name Tetsuo in Shin Megami Tensei Final Story: Atlus Official Replay Novel, Takeshi in Shin Megami Tensei Character Profile: Steven Report I, and Waruo in Kazuma Kaneko Complete Illustration Works.
The Chaos Hero's Illustration Works name Waruo is written in Katakana as ワルオ, which literally means "bad man" (悪男).
The Chaos Hero's coat and glasses are obtainable promotional items in Shin Megami Tensei IMAGINE and are the only gear based on a series design that can be worn by either gender. Like many series sets, it has been released multiple times with various stats and has also received a recolored edition in the green Outlaw Coat. The Chaos Hero's sword and a male-only set designed after his appearance after fusing himself with a demon were also later released.
He is also the only hero the protagonist can use following his transformation, but only for a brief time.
After fusing, the Chaos Hero does not wear his helmet in his close up portrait in the Sega CD version of the game.