Mecha Sonic is Sonic the Hedgehog after undergoing Roboticization. Sonic assumed this guise two times: once after being captured by Dr. Ivo Robotnik, and again in a battle against Dr. Eggman engineered by the Bem.
Mecha Sonic first appeared after Sonic failed to convince the other Knothole Freedom Fighters to allow him to be Roboticized with a Neuro-Overrider device so as to increase his ability to fight Dr. Robotnik and thus ensure his enemy's defeat. Frustrated, Sonic left the group and headed for the Knothole gym, only to be ambushed and knocked unconscious by his old enemy Nack the Weasel, who delivered him to Robotnik in revenge for Sonic's role in his previous capture. Unable to escape, Sonic was then placed in a Roboticizer and transformed into Robotnik's obedient, super-powered slave with orders to destroy Knothole and the Freedom Fighters.
After handily destroying a giant Crabmeat, one of Robotnik's earlier creations, Mecha Sonic flew to the Great Forest and engaged the Freedom Fighters in battle. Despite his old friends' clever countermeasures, Mecha Sonic proved a virtually unstoppable opponent; even Bunnie Rabbot and Knuckles the Echidna failed to stop him. (StH: #39) It was only after Knuckles himself submitted to Roboticization with a Neuro-Overrider in place to maintain his free will that a foe emerged who could challenge Mecha Sonic: Mecha Knuckles. The two engaged in an explosive battle in which much of Knothole and then Robotropolis was leveled, the latter when they crashed into a stockpile of nuclear warheads. Mecha Sonic was left damaged and near death by the blast, and his Mobian personality—which had broken through somewhat before when he had been ordered to destroy Knothole—reasserted itself. Due to the energies of the One Billionth Ring Sonic had collected, his personality remained intact despite the Roboticization, and out of guilt for his actions he asked his friends to let him die. Unwilling to do so, Sally Acorn and her computer Nicole initiated a process that restored Sonic to his healed Mobian form. (SMM)
Following these events, Sonic was tried for treason, as the Freedom Fighters believed that he had disobeyed orders and gone through with the plan anyway. However, after a trip to the Robo Hobo Jungle and finding out where Nack the Weasel was hiding from a group of Badniks, Sonic managed to apprehend Nack and the charges against him were dropped. (StH: #40)
Later, Sonic and Tails were abducted by the Bem along with Dr. Eggman and Snively Robotnik, the former pair undergoing a Roboticization that allowed them to retain their personalities while the latter two were restored to biological forms. The new Mecha Sonic and Mecha Tails were then pitted against their enemies to determine which of them was superior, the stakes being restoration to their previous forms or being trapped in their new forms forever. Unsurprisingly, the Mecha Freedom Fighters overcame their opponents, and thus were restored to their normal states. (StH: #118)
Later, Sonic learned that this was being used as evidence in a trial against a Bem known as Ceneca-9000, who was trying to prove that organic states were better than roboticised states after illegally de-roboticising the entire Planet Mobius. Sonic angrily burst out while viewing the trial that this was unfair, since he could have beaten Eggman as his cute and cuddly self too, and she agreed that it was the Council's idea for the role reversal.
As the Super Genesis Wave was being prepared to wash over both Sonic and Mega Man's universes, Dr. Eggman and Dr. Wily explained to their captured enemies what the wave would allow. Whereas the previous Genesis Waves had allowed only partial manipulation of the worlds from which the doctors hailed, the Super Genesis Wave would allow them to rewrite reality freely. In his vision of a revamped Mobius, Eggman imagined a new Mecha Sonic serving as one of his enforcers alongside Mecha Sally. (StH: #251)
Mecha Sonic commanded Sonic's speed and strength enhanced to incredible levels, as well as granting him a small arsenal of weapons. Both Mecha Sonics could also fly and produce laser beams from their eyes, while the original could electrocute opponents. The original Mecha Sonic had powerful lasers built into his arms, while the second could launch rockets or nets. As robots, Mecha Sonic also commanded optic sensors equipped with heat-seeking and other features, allowing them to lock onto enemies much more effectively than Sonic's biological eyes.
Though the second version of Mecha Sonic had the same personality as the normal Sonic, the first Mecha Sonic had a second personality that was enhanced by Robotnik's Brain-Burn-Thru software which suppressed his normal one for the most part. This personality was unfailingly loyal to Robotnik, and possessed a vehement hatred for the Freedom Fighters.