Unnamed Sonic robot

This unnamed Sonic robot, known as Silver Sonic II among fans, appears in Sonic Adventure and Sonic Adventure DX: Director's Cut. It is one of the many robotic duplicates of Sonic the Hedgehog created by Dr. Eggman. Unlike many of Eggman's robot duplicates though, this unit was never seen deployed.

It was unnamed in its game appearance, although it later appeared in the Archie Comics series and was given the name Silver Sonic II

Sonic Adventure
In Sonic Adventure, and it's remake Sonic Adventure DX, the robot can be spotted inside a stasis tube near Gamma's birth area in the Final Egg. Though it plays no important role in the story. It is largely unknown what happened to this robot, or what was Eggman's purposes to create it.

Archie Comics
In the Sonic the Hedgehog series published by Archie Comics, Silver Sonic II was a scaled-down version of Silver Sonic and a part of the Metal Sonic Series. After the defeat of Perfect Chaos in Station Square, Eggman unleashed Silver Sonic II to attack the vulnerable city. The robot's power proved to be too much for most of the Freedom Fighters, but he was defeated by Sonic, who knocked his head off. He was then reprogrammed and repaired by Nate Morgan to act as the city's defender against Eggman and outside forces of evil.

When Eggman went to retrieve Silver Sonic II's body and repair him, he was chased out of the city. Silver Sonic II chased Eggman all the way to the remote Prison Island, where Eggman fell down a deep hole and was presumed killed. (In fact, Eggman survived, having stumbled onto his grandfather Gerald's laboratory.)

Three months later, when Shadow the Hedgehog came to Station Square to steal the Chaos Emerald located in their bank's safe, Silver Sonic II attempted to stop him. Unfortunately, Shadow used his Chaos Control powers and destroyed the city's defender. Silver Sonic II's remains were picked up by detective J.J. Moto, and the robot has not reappeared since (it could be presumed that he is undergoing reconstruction).

In StH #225, another Silver Sonic was made, this one being called Silver Sonic III.

Trivia

 * The robot is, to date, the only Sonic look-a-like to appear in a game (or two if one counts the Director's cut to be a separate game) to not be fought as a boss or be a playable character (in an official sonic game).
 * Since the robot went on without a name at all in it's original appearance, fans and players have called it "Mecha Sonic", however the Archie comics later on named the robot Silver Sonic II which has since become it's primary name though some still prefer to call it "Mecha Sonic".