He is the Cat that saves the day during the "Awefull Battle of the Pekes and the Pollicles" by showing his eyes ("like fireballs, fearfully blazing) and flashing his jaws ("amazing!") to the Pekes and the Pollicles and scaring them off just before the Big Police Dog came back to his beat. Munkustrap doesn't seem to fond of him, though... he gets really exasperated because the Great Rumpus Cat doesn't learn his part well enough and keeps messing up!
I read about something that gave me a new respect for ole Rumpus. In the scene where he catapults from "a small basement flat" and crashes to the ground, he's projected from a springboard that sends him through 2-3 inches of stage! If he gets off even a couple of inches from the center of the springboard, his arms could be ripped off with the sheer force of the impact! In some places, it's illegal because it's so dangerous. Wow!