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Metalhead first came in episode Metalhead.Donatello made him and they used him to fight enemies. He was once took over by Kraang and tried to destroy the turtles, but Donnie killed his power systems. Later on, Donnie remade him to drive the Shellraiser to distract the Kraang while the turtles tried to get into the portal. After Donnie fixes him, again, he held open the Kraang Portal as the turtles got the mutants out.
Metalhead is unlocked once you complete the level 'In and Out again' where he hacks the computers in the aquarium to open a door for Leatherhead to get out and other things. He also appears in the final battle being stopped by Shredder, even though you don't play as him. He still parties in 'Everybody do your share'.