Sly is the brother of Ty, the main character of the game Ty the Tasmanian Tiger. He is an anthropomorphic Thylacine (Tasmanian tiger) and served as a recurring antagonist until he realized the truth of the situation.
Ty the Tasmanian Tiger
Sly was raised by Boss Cass to become a master attacker and henchman, all while claiming that he had been abandoned by his parents. (This was despite Cass' prejudice towards mammalian life, although it never stopped him from gloating about his plan to exterminate and displace mammals in Sly's presence.) Cass was attempting to gather the Talismans responsible for opening a gate to the Dreaming (an otherworld containing the Bunyips that watch over Ty's home) where he intended to banish all mammalian life. To meet these ends, Sly worked for Cass, and tried to kill Ty on several occasions on Cass' orders. However, Ty proved himself to be "more of a challenge" to the Tasmanian tiger.
During their final confrontation in Cass' base, which contained lava flows, Ty disarmed Sly's boomerangs. This knocked him over the edge although Ty saved him, which surprised Sly. Still acting as henchman, Sly vowed that Ty would regret saving his life. However, Sly learned the truth about Cass having imprisoned his and Ty's parents in the Dreaming. Enraged, Sly rebelled against Cass and threw his Doomerang for Ty to use against Cass' machine. Cass defeated and his machine destroyed, Ty reacquired the final Talisman in Cass' possession and was able to bring back their parents from the Dreaming.
Post-Ty the Tasmanian Tiger
In the 2 other games, Sly became one of Ty's allies. He also opened up a shop called Sly's Shack, selling boomerangs that his brother could buy for future battles.