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Darkdeath Evilman is the main antagonist of Zettai Hero Project: Unlosing Ranger Vs. Darkdeath Evilman, a Roguelike published by Nippon Ichi software in 2010. The game centers around a young man (named by the player), who is chosen to become the next "Unlosing Ranger" after Pirohiko, the previous Unlosing Ranger, was ran over by a car.The meat of the game centers around the the main character training for the final encounter with Darkdeath Evilman in "Bizzaro World", a moster-filled parralel universe where the Unlosing Ranger can solve people's problems by beating up the physical manifestations of those problems. Every level completed leads to a battle with the titular villain, though for the most part these are simply glorified cutscenes with limited player interaction. There is a proper final boss between the main character and the Bizzaro version of Darkdeath Evilman.
Late into the game, it is revealed that Darkdeath Evilman was a child prodigy who was kidnapped by an evil organization. Since nobody came to save him, he lost faith in heroes. As a result of this, he concocted a nihilistic plan where he would first kill the Superbaby, and then blow up the earth with a nuclear warhead.
Luckily, the Unlosing Ranger and the Reckless Cop Dangerama were inspiring enough to win over Darkdeath Evilman. The game's ending shows that he has given up his villainous way for good, and has joined up with the Unlosing Ranger and his friends (though the game still refers to him as "Darkdeath Evilman".
Disgaea Dimension 2: A Brighter Darkness
Darkdeath Evilman is a postgame boss character is Disgaea Dimension 2. He is summoned by the Krichevskoy Group to do battle with Laharl, and finally pledge their loyalty to the prince once Darkdeath is defeated. Darkdeath does not talk at any point during this sequence, so it's unclear whether this is the real Darkdeath Evilman or just his robotic shell.
Disgaea 5: Alliance of Vengeance
A monochrome variant known as "Proto Darkdeath" shows up as an optional boss in Disgaea 5: Alliance of Vengence. He appears in the item world at random intervals, but an assembly bill can be passed that forces him to be summoned.