Marvell is the replacement janitor that was hired when Janitor was fired, until he was fired by Janitor who was looking to get rehired.
When Dr. Maddox fired Janitor for hurting J.D., she hired a replacement to do his job for him. J.D. and Carla were surprised that he was hired. He is the complete opposite of Janitor as when he arrives he says a welcoming "hello" to J.D. and tells him if there's anything that needs cleaning up, to give him a shout. ("My Jerks")
After Dr. Maddox was fired, under the guise of Dr. Jan Itor, Janitor told Marvell that he was fired. ("My Happy Place")