Sonic DC - Metal Sonic

Metal Sonic is an evil robotic version of Sonic created by Dr. Robotnik. He first appears in Sonic the Hedgehog CD.  Metal Sonic notably acted as the main antagonist in Sonic Heroes, appearing in a new form as Neo Metal Sonic before transforming into the game's final boss, the Metal Overlord.

He also appears as the main antagonist in the Sonic the Hedgehog (OVA), where Eggman records Sonic's abilities and uploads them to Metal Sonic, who proceeds in attempting to destroy the world before being tossed into lava by Sonic.

Collecting all the emblems in Sonic Adventure DX: Director's Cut unlocks Metal Sonic as a bonus playable character in Sonic's stages, and purchasing both episodes of Sonic the Hedgehog 4 unlocks bonus levels in which Metal Sonic is playable.