According to the book, Roid is an alter-ego of Munch, both mentally and physically. Like a split personality, but one that takes an actual form. This metamorphosis was caused by Vykker-made steriods; Munch was originally supposed to be a steroid test suject in addition to what they did to him in the game.
It's like the Hulk: when Munch gets pissed enough, he changes into the form of Roid, which is a complete paralell to Munch. Munch is small, kind, reserved, and quiet, while Roid is large, angry, noisy, and violent. This change from Munch to Roid also gives Munch super strength and invulnerability. It wears off over time, though.
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