He is an evil, ultra-high performance, very advanced Badnik version of Sonic the Hedgehog that was created as a secret weapon by Dr. Eggman for the sole purpose of destroying the hedgehog. Modeled with extreme accuracy after Sonic, Metal Sonic not only resembles his template, but can also imitate his moves and achieve speeds matching and even exceeding the hedgehog's own.
While usually serving as Eggman's obedient and silent enforcer, Metal Sonic is an intelligent robot with a very dark side. A cold-hearted, aggressive, and ruthless killer, he is obsessed with proving his superiority to Sonic and eliminating him for good to the point where he has rebelled against Eggman more than once to achieve his goal.