Sasuke is the loyal and abused man-servant of the Kuno family. Bound to the family by his twisted sense of honour, Sasuke defends them and serves them to the best of his abilites. Many of his tasks are utterly demeaning and cruel, such as being required to make shadow puppets so that his master Kuno can fall a sleep, being used as a wash towel by Kuno, target for both Kuno and Kodachi's attacks. Sasuke is a sneaky little man, and often is sent out by Kuno or Kodachi on missions to try aid their personal pursuits (i.e. their infatuation with Ranma-chan and Ranma-kun) It was Sasuke that discovered Ranma's sole weakness, cats. He is rather enamored of Akane Tendo, mostly because she is so kind to him. (that and she ignored the fact that he still wets the bed, unlike Kodachi who uses the fact to blackmail him)
Sasuke Sarugakure is an anime-exclusive character.
Sources:
caboodle.tripod.com/ranma/sasubio.html
Wikipedia