In the MUGEN modding subculture, "Theory" or "Mercenary" often refers to custom, highly edited, or over-the-top character edits. These builds feature modified AI, custom sprite work, visual effect overhauls, and alternate movesets for classic KOF mercenaries like Ralf Jones, Clark Still, Heidern, Whip, or Leona Heidern.

Disclaimer: Mugen games are often fan-made projects that use copyrighted characters. Ensure you follow local regulations regarding the use of such software.