He was Boss Cass' best henchman in Ty 1, where he was featured as a mini-boss. Near the end of the game, Ty rescued him from falling into lava. He told Ty that he would regret saving him before running off. During the final battle, he showed up and gave Ty a helping hand in defeating Boss Cass, and ended up as a close ally by the time of Ty 2.
Sly started out on Boss Cass's side, as Cass had raised him and taught him that his parents had left him. But Sly changed sides when he found out the truth; Cass had trapped Sly's and Ty's parents in the Dreaming in the first place.
Sometime in between the events of Ty 1 and 2, it was revealed to both Ty and Sly that they were close brothers. It is not known at which point this was revealed, but whenever Ty approaches Sly in Ty 2 or 3, he would say something along the lines like, "Hey bro!" or when he gets hit by Ty's rangs in Ty 3, he would say,"Uncool, bro!"
In Ty 1, he serves as a mini-boss of the game.
In Ty 2, he helps Ty in the beginning of the game and at the end, when they go to Cass's hideout. He also owns Sly's Shack, a shop where he sells upgraded boomerangs.
In Ty 3 he is an ally to Ty and Bush Rescue, he meets Ty at the beginning of several missions. Sly's personality is more easily seen in this title due to a greater role in the story. In particular, he is annoyed at doing work for Boss Cass, saying it reminds him of his past. He, Shazza and Fluffy attempt to assist Ty in taking on the Quinking, but their plane is shot down by enemy fire before they can get far. Ty goes on ahead anyway, as the Redback Gang arrive to help out the other three.
By the time of Boomerang Blast, his outfit has taken on a more sinister apperance, complete with trenchcoat and footwear/gloves that accent his claws. He also seems to have had an accident, now sporting a scar across his muzzle.
In Ty the Tasmanian Tiger 2013, Sly is not seen for much of the game, having gone missing. Soon, a mysterious figure similar to Sly is seen setting numerous parts of Southern Rivers on fire, before kidnapping Ranger Ken; Ty's investigation reveals that it is in fact Sly behind the crimes. Sly attempts to murder his brother through various traps, though Ty is able to save Ranger Ken, albeit allowing Sly to escape in the process. Sly professes he has come back to his senses and says he should not rely on his friends, unlike the weak Ty; Ty proceeds to chase Sly's crabmersible down while riding on Bull's back. Bull rams Sly's vehicle so hard it explodes. Ty is naturally full of grief at Sly's death... until he sees that it was just a robot all along, indicating the real Sly is still out there somewhere. Ty eventually manages to rescue the real Sly from Cassablanca, leaving him behind so he may go deal with Boss Cass.
In Ty 1, Sly clearly had evil, thuggish tendencies, indulging in evil laughter, being fiercely loyal to Cass' cause, and trying to kill Ty more than once. When Ty saved him from falling to his death, he was noticeably shocked, having difficulty coming up for words for such kindness. Sly is apparently good at information collection, since shortly afterward he turns on Boss Cass, stating that it was because Cass had lied to him. This would indicate he has a vengeful streak, as Cass had been his adoptive father prior to this.
By Ty 2, he had reformed considerably (likely as a result of finding out he had a family after all), though he still made boomerangs of destructive power as a hobby. Though he had changed allegiance, he still remained fairly aggressive, threatening to clobber Ty if accidentally hit. Though he is confident at getting a chance at vengeance on Cass, he is noticeably disheartened at seeing Ty having a better Bunyip than himself. His desire for vengeance seems to not be too overpowering, as well, given that he agreed to let Ty do the work of fighting Cass while he and Shazza rescued the villagers.
In Ty 3, the once menacing Sly had become fairly heroic and much more easygoing. He became playful around Ty, sharing jokes with him, showing he had softened up considerably. He clearly shows regrets for what he did working for Cass, saying that helping Cass out reminds him of all the bad things he did in the past; he does not specify exactly what he means, however. He has bonded quite closely with his brother, saying he would protect Shazza for him should Ty perish.
His movements are also very erratic, as if anxious, constantly shifting and moving his limbs about. This may be related to Ty having been presumed dead for the past six months, and all the stress that has resulted from it and the radically changed environment around him.