He is a lab rat on the verge of death, granted a chance to relive his final day in order to fulfill his life's dream and die happily. In his case, taking revenge on his human scientist owner. As in, killing him. He has a gaping, cartoonish, circular hole in their stomach, which shows Mad Rat’s ribs and a red, floating heart in the middle of it. As for personality: He seems to have a bit of a temper, doesn’t exactly open up to people all that much, and can/will fight by himself if needed.