Metal Sonic (also known as Mecha Sonic, and also Metallix amongst some fans in the UK and Metalla in Japan) is the "robotic counterpart" of video game character Sonic the Hedgehog. Being a robot, Metal Sonic has no gender; however, Metal is usually referred to as being male, since after all he is a robot based off a male character. As well as resembling Sonic, he can also perform Sonic's 'spin dash attack'.
A prototype first appeared in Sonic the Hedgehog 2 as one of the final bosses. This prototype did not have Metal's painted blue coat, and was also very clunky compared to the sleeker and generally more Sonic-like Metal.
His next appearance was in Sonic CD for the Sega CD console. In this game, he is under the control of Dr. Eggman, the main antagonist of the Sonic games. Metal Sonic is sent back in time to change the past so that Dr. Eggman can rule the future. Later, he kidnaps Amy Rose, who had been following Sonic. He was defeated in the Stardust Speedway zone.
Another time one of Metal Sonic's incarnations appeared was in Sonic & Knuckles. He appeared in Sonic and/or Tails' route as a midlevel and final boss in Sky Sanctuary Zone, and as a final game boss for Knuckles in the same place. This incarnation looked a little different from the Metal who appeared in Sonic CD and subsequently in other games. He appeared to be taller, and the construction of his limbs and face looked different.
Against Sonic and Tails, this Metal incarnation appeared three times in the level - twice in machines Dr. Eggman had used in Sonic the Hedgehog and Sonic the Hedgehog 2 and once in a one-on-one fight. Against Knuckles, he fought one-on-one as in the final fight with Sonic and Tails, but in the second phase of the battle he used the Master Emerald to go Super (much like Sonic can go Super and become Super Sonic) for brief periods of time.
His second major appearance was in Knuckles' Chaotix, where he absorbs the Chaos Rings' powers to wreak havoc in the Carnival Zone, but he was defeated by the Chaotix before he could.
He also starred in Sonic Anime, being Sonic's main rival in it. In the anime, they had a mindlink; that is, they were able to hear each other's thoughts. Tails reprogrammed him near the end to be on Sonic's side, but Metal was destroyed soon after.
He then had appearances in Sonic Drift 2, Sonic the Fighters, and Sonic R, as well as a cameo in Sonic Adventure, inside a tube in one of Dr. Robotnik's hideouts.
His next appearance was as a multiplayer character in the GameCube remake of Sonic Adventure 2, called Sonic Adventure 2: Battle. He can project a black shield to protect himself from the attacks of other characters, but has no special attacks himself. After that, he had a cameo as an opponent in Sonic Pinball Party's Story Mode. After that, he appeared as a gift for getting all of the Emblems in the GameCube remake of Sonic Adventure, called Sonic Adventure DX: Directors Cut, one could play time trials as Metal Sonic in Sonic's stages. Metal Sonic essentially acted as an alternate skin for Sonic.
His latest appearance is as the main antagonist in Sonic Heroes. Assuming the guise of Dr. Eggman, he copies the physical data of Sonic, Shadow and Chocola (a chao that was sought by Team Rose) to assume his ultimate form, Metal Madness, and later Metal Overlord. This transformation was made possible with the power of Chaos, the water-like creature from Sonic Adventure. This not only increased Metal Sonic's power, but gave him shape-shifting abilities and a new look. How he managed to get this power is never explained, but after he is defeated by Team Super Sonic in his Metal Overlord form, he reverts to his original form.
It is not known if he will appear again.
The Brotherhood of Metallix
Over in the UK's official Sonic publication, Sonic the Comic, Metal Sonic was introduced in the magazine's adaptation of Sonic CD, named "The Sonic Terminator." Relegating "Metal Sonic" to the character's title, Sonic's robot double was redubbed with the less-clunky moniker of "Metallix" (hence, he was "Metallix, the Metal Sonic"). Metallix lured Sonic to the Miracle Planet by capturing Amy Rose, and in the course of the adventure, Sonic travelled back in time to prevent Doctor Robotnik from cyberforming the planet, thereby erasing Metallix, who used the planet as a power source, from the timeline.
A new model of Metallix appeared during the comic's adaptation of Sonic and Knuckles, stealing the Master Emerald for Robotnik to use as the power core of his new Death Egg. Metallix drew power from the emerald and incapacitated Knuckles, but Sonic was able to destroy him by using the emerald's power to become Super Sonic. Before he died, the new Metallix revealed that both he and the original Metallix were members of an elite brotherhood.
This brotherhood soon revealed itself when Knuckles was accidentally transported into the Special Zone and encountered Chaotix. Operating out of Robotnik's abandoned Egg Fortress in the Special Zone, the Brotherhood of Metallix - innumerable Metallixes identical to the original, ruled over by an giant red and gold Emperor - had been charged with producing the ultimate Badnik version of Sonic, but had developed their own plans and goals. With the help of the turncoat Chaotix member, Nack the Weasel, the Brotherhood of Metallix captured the Omni-Viewer, Chaotix's sentient, space-time transporter. With Knuckles's help, Chaotix were able to retrieve the Omni-Viewer, and though it appeared that the Metallixes had been defeated, they in fact survived, and had copied the Omni-Viewer for their own purposes.
In their next move, the Brotherhood captured Grimer, Doctor Robotnik's chief scientist, who had created the Metallix design in the first place. They charged him with building the Alpha Device, a device that began to cyberform the Miracle Planet once again. Sonic's attempt to stop the plan apparently succeeded, but his ally, Porker Lewis, was trapped on the Miracle Planet when it vanished from Mobius orbit in it's usual pattern. During that month, the Metallixes reasserted themselves, captured Porker and completed the cyberforming of the planet. A Metallix, disguised as Porker, was left to stand guard as the Metallixes departed into Mobius's past with their copy of the Omni-Viewer, and when they returned, they encountered Sonic and Chaotix, who had come to investigate. But then the terrifying truth became apparent - the Metallixes had changed history, and Mobius had become a city-planet, covered with filthy industry, where surviving Mobians lived underground. And the reason for the change soon became apparent, as Sonic and Chaotix attempted to fix the timeline by using the real Omni-Viewer - the Brotherhood of Metallix had travelled back in time to the point of Doctor Robotnik's creation, and removed from the fridge the rotten egg that would transform Ovi Kintobor into Ivo Robotnik. Kintobor never became Robotnik, and the Metallixes, created by Grimer alone, ruled the planet. Retrieving the egg, Sonic and Chaotix returned to Kintobor's lab and replaced it, and then, as Kintobor held it in his hand, Sonic yanked a cable to trip him up, triggering the chain of events that turned him into Doctor Robotnik. Despite the disturbing revelation that he was responsible for creating the greatest evil on Mobius, Sonic returned to the present with Chaotix, where the robots still held control of the Miracle Planet. Forced into an alliance with Doctor Robotnik, Sonic retrieved from the villain a device that deactivated all the Metallixes in a pitched final battle.