|First appearance||Conker's Bad Fur Day|
|Eye color||Black (Bad Fur Day)|
Light blue (Live & Reloaded)
- "Oh no, I think I had too much!"
- — Marvin
Marvin has brown fur, a white belly, white fur on the tip of his paws and near his tail and three whiskers on each side. He also has buckteeth and a few hairs on his head. Despite being a mouse, Marvin is bipedal. However, he does run around on all fours, but walks with just two. Marvin has black pupils, which were recolored to light blue in Live & Reloaded.
Conker meets Jack at the start of Barn Boys. The Large Female Box is sitting on him because Marvin is near them, and she has musophobia. Conker must fetch a block of Cheese and feed it to Marvin. After doing so, Marvin requests another Cheese. Marvin's weight grows from every Cheese he consumes, and he frequently burps and farts out light green gas clouds. Conker vomits if he stands at the gas (he does not vomit in Live & Reloaded). The cycle repeats twice, as Conker must feed Marvin two more blocks of Cheese.
After being fed a third block of Cheese, Marvin bloats, and his belly turns into a large, fat sphere. Marvin realizes he ate too much Cheese and explodes; his body parts and guts fly everywhere. His mutilated tail lets out a final fart. Marvin still blinks his eyes after blowing up, which implies that he survived. Marvin's tail is the only body part that remains on the ground after the cutscene.
Marvin was confirmed to have survived after Conker defeats Heinrich in the throne room. After Franky introduces the rest of the characters, Marvin is the first to come in. His body is sewn together, and he has normal weight. After entering, Marvin notices a block of Cheese and chases it. He then stands at Conker's throne, which he also does in the opening cutscene.