Sasuke is basically the Kuno family's punching bag—a loyal servant bound to them by some twisted code of honor that keeps him coming back for more abuse. His job description includes all the worst stuff you can imagine: making shadow puppets to help Kuno fall asleep, getting used as a literal towel, and serving as a convenient target for both Kuno and his sister Kodachi to practice their attacks on. He's sneaky though, and the family regularly sends him out on missions to help with their various schemes, mostly their obsessive pursuits of Ranma in both forms.
Sasuke's actually useful too—he's the one who figured out Ranma's weakness is cats, which became pretty important intel. Despite all the crap he takes, he's genuinely attached to Akane Tendo, mostly because she's actually nice to him. She doesn't hold his bedwetting problem over his head like Kodachi does, which probably means more to him than it should.
Anime-exclusive character.
Content compiled by AnimeList.moe from publicly available sources.