Silver was an Ice Devil Slayer. Silver Fullbuster is the father of Fairy Tail Mage, Gray Fullbuster. Silver died in X774 along with his wife, Mika.
Not much is known about Silver's past other than that he supposedly died during Deliora's attack on his village. At one point, however, Silver was seen laughing with his son, encouraging him to become strong. After his death, Silver's corpse was stolen by Keyes for experimentation, among many others. Keyes' experiment succeeded, and for seventeen years, Silver "lived" as a normal human would.
However, Silver used his new "life" to exact revenge on the Demons that took his family from him. He masqueraded as a loyal member of Tartaros, having been inducted into the guild by Mard Geer. Silver learned Devil Slayer Magic and covertly killed any Demon he encountered, all in preparation for his ultimate plan: the destruction of Tartaros.
After seeing that his son was alive and witnessing Gray's performance at the Grand Magic Games, Silver decided to abandon his plan. He realized he didn't have the right to fight for Gray and his deceased mother, as his own hands were too dirty, presumably from his missions for Tartaros.
(Source: Fairy Tail Wiki)
