User blog comment:Loolveus/Unexpectedly Dark Cartoon Villains/@comment-30634893-20171028154733

A bit recent, but Shadowy Figure for sure. Up until "T.K.O.", all the villains were just goofs, but then, right out of nowhere, comes a guy who manipulates K.O. into unleashing his darker self and nearly murdering his friends and later abuses him both psychologically and physically to boot. That, and he's the only villain in the series to be treated with complete seriousness all the time.

Given the nature of the show, he might be seen as less dark or evil overtime, but he definately qualifies as he stands now.