Things weren't going well for
Dr. Robotnik after his defeat in
Sonic the Hedgehog 2. His first attempt at a mechanical
Sonic the Hedgehog clone,
Silver Sonic, was a failure, having been destroyed by the blue blur. What's a mad doctor to do?
He creates Metal Sonic, that's what.
Metal Sonic was designed to be a mechanical duplicate of Sonic the Hedgehog. The robot was to equal (and even exceed) Sonic's speed, agility, and attacking prowess. He first appeared in Sonic CD when he kidnapped Amy Rose and held her hostage as the prize in a deadly race. Sonic outraced him, however, and the robot slammed into a spiked wall. Oops.
He appeared again at the end of Sonic 3 and Knuckles. He was sent to stop Knuckles from stealing back the Master Emerald from Dr. Robotnik. Metal Sonic had been upgraded and was now able to use the Master Emerald to power himself up in the same way Sonic and friends can draw power from the Chaos Emeralds. In the end, however, Knuckles sent him to the scrap heap. When playing as Sonic Metal Sonic imitates classic boss attacks from previous Sonic the Hedgehog games during the Sky Sanctuary Zone.
After being repaired yet again, Dr. Robotnik sent him to steal the Chaos Rings from Carnival Island. However the upgrades turned out to be less than effective and he was defeated yet again. (SEE: Knuckles Chaotix)
One more important ability Metal Sonic possesses is to imitate the attacks of his enemies. According to the SonicHQ website, attacks Metal Sonic could use included Spin Attack, Propeller Dash, Knuckle Glider, Magic Hammer, Tongue Attack, Bomb Attack, Shoulder Throw, and Super Spin.
Metal Sonic has appeared in the following games:
For now, Metal Sonic rests within a tube in Dr. Robotnik's lab. However, I seriously doubt this is the last we'll see of him. You can't keep a good badnik down, after all.
SonicHQ source: http://www.sonichq.org/general/profiles/index.php/top.a/bottom.m/category.sega/type.character/index.10