Marvin as a zombie is much like Forest Speyer from the first Resident Evil. He can be faster than other zombies but can still be killed with a single shotgun blast to the head.
In Resident Evil Outbreak File #2, Marvin appears as an NPC fighting off zombies inside the front gate of the Raccoon Police Station. Here he appears already injured. Unlike the other officers fighting throughout the police station, he is not killed after enough time in the scenario has passed.
Marvin is also unlockable as playable in the Outbreak games. He can be unlocked by completing the "Desperate Times" Scenario as Kevin or a Kevin-type character on any difficulty. He can then be purchased for 4000pts. He can be made available in File 1 by use of File #2's data convert feature.
There is another version of Marvin in the Outbreak games. This version is coded as a character as well but was never meant to be played as, as it is statless and must be modified to have stats via game enhancer. This version is the injured Marvin's gameplay model and features a patch of blood on his stomach.