Metal Sonic is a robotic version of Sonic created by Dr. Eggman. (also known as Hyper Metal Sonic in the 2 episode OVA anime) He first appears in Sonic the Hedgehog CD, with orders to change the past by Eggman. He returns in Knuckles' Chaotix, where he attempts to obtain the Chaos Rings, but he is stopped by the Chaotix. He appears as a boss in Sonic Triple Trouble and Sonic the Fighters. He is playable in Sonic Drift 2 and Sonic R, and a bonus character in Sonic Adventure and Sonic Adventure 2. He acts as the main antagonist of Sonic Heroes, where he transforms himself into Neo Metal Sonic. His appearance changes vastly and he uses an ability to transform into other beings in order to manipulate the main characters, copying data from Sonic, Knuckles and Tails, the Ultimate Lifeform data from Shadow and the Chaos data from Chocola Chao and Froggy. He betrays Eggman, and uses the collected data to transform into a large mechanical dragon called Metal Madness or Metal Overlord, the reason for his actions were so he could complete his goal to destroy Sonic and to create a robotic empire. After being defeated, he contemplates not being able to defeat Sonic, returns to his original form, and he is taken away by Omega and Shadow. Eggman reobtains him and reprograms him to be obedient. He returns as an enemy and unlockable character in Sonic Rivals, and appears as a playable character in Sonic Rivals 2, acting under Eggman's orders.

Metal Sonic is able to match the speed of Sonic by using an engine to propel himself. He also has several of Sonic's abilities, along with being able to copy the abilities of others, such as Knuckles' strength and Silver's psychokinesis powers. He usually only communicates with a series of electronic noises, though he is able to speak in his Neo form. The form also grants him the ability to transform into the large "Metal Madness" and "Metal Overlord" forms after collecting enough data, which grants him various projectiles and the endurance to match Super Sonic. In Sonic Rivals 2 Eggman Nega creates a black and yellow copy of Metal Sonic called Metal Sonic 3.0. Metal Sonic plays a primary role in Sonic The Movie, acting as a major villain, built by Dr. Robotnik to destroy Sonic. Sonic and Metal Sonic share a mental bond, capable of feeling the same thing as each other, both physically and emotionally. Metal Sonic ultimately perishes in the conclusion of the movie, melting in a laval pool, deciding there is "only one Sonic." Metal Sonic is voiced by Junichi Kanemaru in the Japanese version of Sonic Heroes and Ryan Drummond in the English version.

(Source: Wikipedia)