Jeff

Jeff is, according to himself, Mario's arch-nemesis. He first appears in Mario vs Jeff, in which he taunts Mario throughout the episode. Mario repeatedly attempts to punch Jeff, but as he frequently references, Jeff is omniscient and cannot be hit by the plumber's attack.

Personality
From what can be seen, Jeff is maniacal and evil. He frequently laughs repeatedly whenever Mario takes damage and calls Mario's deaths hilarious.

Trivia

 * Jeff is the only character with a voice actor. He is voiced by Pseudodino1. He is also the only character not to appear on-screen.