Jeffy Jeffy, known simply as Jeffy, is a secondary antagonist, later turned protagonist of the SuperMarioLogan franchise. Known for being a foul mouthed, dim-witted, weird, ill-behaved, dumb, perverted young teenager with an unusual personality and a strange choice of clothing; he is the adoptive son of Mario and his wife Rosalina.
- Lance Thirtyacre
- Anthony Miller (formerly; animal noises)
- Chris Netherton (Jeffy's Brain! French accent and Old Jeffy in Baby Jeffy!)
- Chilly Jimenez (Jeffy's BIG Easter!)
- Tito Jimenez (adult)
- Jeffy plays as Gromit in Mario & Jeffy: A Grand Day Out, Mario & Jeffy: The Wrong Trousers, Mario & Jeffy: A Close Shave, Mario & Jeffy: The Curse of the Were-Dinosaur and Mario & Jeffy: A Matter of Loaf and Death