The Rat ManThe Rat Man only appeared in episode 22 of season one, titled The Sewer Thing. The Rat Man's real name is Al Rizzo and he was the janitor at General Splice Labs. When Dr. Roberta Frisk, Dr. Trever Sindler and Dr. Maurice Muncy were working together, a beaker full of a strange formula that was created with rat DNA was knocked over. The formula leaked into an air vent and leaked into Al's room and oozed right into his bed. The formula mutated him while he was sleeping and slowly turned him into a giant rat. Al started to get his revenge by gathering up the scientists that changed him into the rat man and turn them into giant rats themselves. Lukily for Al, Muncy was able to explain to Martin how to create an antidote by mixing Al's DNA (his hair which was pretty much the only human left in the rat man) with a sample of the original formula and heating it.
The DNA seemed to give Al abilities of a rat including the ability to climb walls, chew threw anything and colapse his spine. Also his spit seems to be able to cause a person to pass out for a small amount of time. If someone gets bitten by the rat man they slowly get turned into a rat themselves.