Silver Sonic MK I
The original Silver Sonic was designed by Robotnik Prime using data he had gathered on Sonic during his brief tenure as Mecha Sonic. When the first Death Egg was launched, Silver Sonic was assigned to be the station's primary protector.
When Sonic and Tails, who had stowed away aboard the Death Egg in an effort to destroy it from the inside, tried to escape as the vessel they found themselves cornered by the hulking Silver Sonic.
Unlike Sonic, however, Silver Sonic was not designed for speed and could not move as fast as its counterpart but what it lacked in speed it made up for in raw strength. The two did battle, which ultimately ended with Sonic managing to short circuit Silver Sonic using a broken power cable before the titanic doppelganger could crush him beneath its boot.
Sonic then used Silver Sonic's body as armor to combat Robotnik, who had donned a suit that enhanced his powers thanks to its direct connection to the Death Egg's power core. Later, just before the Death Egg exploded, Sonic grabbed Tails and a few power rings and leaped off of the station, upon landing Sonic discarded the remains of Silver Sonic which were never recovered by Robotnik or anyone for that matter.
Silver Sonic MK II
The second Silver Sonic was constructed by Doctor Eggman and, as part of a revenge scheme for ruining his plans involving Perfect Chaos, was unleashed upon Station Square during a farewell celebration for Sonic and his friends.
Unlike it's giant and clunky predecessor, the new Silver Sonic was sleek, better equipped and faster than a speeding bullet. It had also been given a personality that mimicked Sonic's own.
After firing a pair of guided missiles on Sonic, Silver Sonic engaged the Freedom Fighters in combat until Sonic was able to enter the fray and plow the war machine into a large building complex with an appliance store on the ground floor. Using one of its metallic whips, the insane robot ripped the main support column apart and brought the entire building down on top of them both.
Silver Sonic emerged from the rubble, believing his counterpart to be dead, and went on a rampage through Station Square completely unaware that Sonic had actually burrowed underground before the building could crush him. Silver Sonic was surprised to learn Sonic was still alive when the blue hedgehog emerged from a manhole and grabbed onto him.
As Silver Sonic flew across the city with his organic counterpart in tow, he activated his built in mini-laser cannon and tried blowing off Sonic's head, but Sonic was able to leap off of Silver Sonic fast enough that the robot ended up blasting his own foot off. When said foot knocked Tails, who had saved Sonic from a lethal fall, unconscious Sonic's anger grew and he spin-dashed into Silver Sonic and ripped his head off.
When Eggman went looking for Silver Sonic in Station Square, he found that his robot had been repaired and reprogrammed to serve as Station Square's new G.U.N. approved protector. Silver Sonic pursued Eggman, who was forced to run on foot thanks to the destruction of his Egg Carrier, all the way into the forests beyond Station Square's borders but stopped when Eggman fell into a pit and could no longer be detected by his former creation.
Weeks later, when the newly reawakened Shadow the Hedgehog broke into a bank vault and stole the Chaos Emerald within, he was confronted by Silver Sonic who immediately engaged him in battle. However, Shadow proved to be too powerful and damaged Silver Sonic beyond G.U.N.'s ability to repair.
Silver Sonic MK III
Like the original, the third Silver Sonic was a titanic juggernaut whose primary function was to defend the Death Egg MK II from intruders. Unlike the original, the MK III was a perfect blend of the previous units, combining the size and power of the original with the speed and maneuverability the second possessed.
It made its presence known when Sally and Sonic infiltrated the Death Egg in search of Eggman by tearing its way out of a tiny elevator shaft. While the sight of it horrified Sally, Sonic was more than excited to combat the newest model of his rarely seen doppelganger.
While the two hedgehogs engaged each other in an all-out brawl, Sally decided to proceed further into the Death Egg only to be blasted to pieces by a hidden Sentry Gun. When Sonic heard the Sentry Gun fire, he turned away from his gigantic counterpart and began running to the source of the gunshots.
Silver Sonic managed to strike Sonic before the Genesis Wave enveloped the world and temporarily erased him from existence. When everything returned to normal, time had rewound back ten seconds before Sally was struck down by the Sentry Gun. Sonic immediately disengaged his fight with Silver Sonic and saved Sally.
Silver Sonic, who had been in hot pursuit of his counterpart, ended up being blown apart by the Sentry Gun instead of the young princess. Later, during Sonic's battle with the latest Metal Sonic, a heavily damaged but still functioning Silver Sonic entered the fray and helped its "little brother" in the fight against their most hated enemy.
However, when Sally managed to invert the World Roboticizer beam to her position beneath it, Silver Sonic and Metal Sonic were exposed to the roboticization process and exploded taking the World Roboticizer and a huge portion of the Death Egg with them. Silver Sonic's remains were dumped out of the Death Egg when Eggman had Mecha Sally toss Sonic off the Death Egg.
Silver Sonic v3.1
This Silver Sonic was simply another MK III model who was dispatched along with Mecha Sally and an army of EggSwats to a village in Feral Forest in an attempt to assassinate Sally's brother, Elias.
When Sonic attempted to confront Mecha Sally, she brushed him off and ordered Silver Sonic to attack, who erupted forth from a nearby hut and immediately engaged Sonic in battle.
While Mecha Sally headed off to finish their mission objective, Silver Sonic tore the village asunder as it tried to kill Sonic, not even caring that it had destroyed a large group of EggSwats and freed several prisoners from their grasp.
But just as Sonic prepared to fight Silver Sonic in earnest, T-Pup came between the two of them and shocked Silver Sonic into stasis mode which allowed the villagers enough time to tie it down and destroy it.
Silver Sonic was later stripped down and its parts along with those of the fallen EggSwats were used to rebuild the village.