Timothy was once a Turtles fanboy and superhero "wannabe" that called himself The Pulverizer. He was lazy, arrogant and impatient, often getting himself into trouble from which the Turtles had to extricate him. But a series of mutations turned him into Mutagen Man—a jar of ooze and visible organs with acidic arms and legs.
Now he requires ooze to regenerate. He is still uncontrollable, so Donnie has frozen him until a cure can be found.