Ai Rin's black and yellow design, as well as her fighting style and use of nunchaku, is likely a reference to Bruce Lee and his film Game of Death.
Contributed by DrakeVagabond
Blacker Baron's design and personality are largely based on Bob Sapp, an American wrestler that was popular in Japan. The design of his weapon "Super Sexy Fists of Fire" is based on American rapper, Ghostface Killah, whose rap persona is a powerful crime lord and pimp.
Contributed by DrakeVagabond
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The enemy "Cybrid Joe" is a nod to an enemy from God Hand "Tiger Joe". Outside of having similar names, they both have identical animations and move sets. Many of the developers at Platinum Games came from Clover Studio, who made God Hand.
Contributed by Hipserious
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There's a cutscene in which Jack and Max are fighting and for a moment the game cuts to a strict black and white (with a little red) color palette. This is a reference to the game's spiritual predecessor, MadWorld.
Contributed by Hipserious