Metalhead

Built as a weapon against the Turtles, programmed with all four of their brainwaves
Managed to overpower the Turtles, reprogrammed by Donatello to fight for them
Exhibiting different personality traits from all four Turtles
Shot by Bebop, leading to a malfunction that made him attack everyone, was turned off
Eventually fixed, joining the Turtles in their fight against Krang & Shredder