Sasuke is a ninja in service to Sanada Yukimura, and thus also to Takeda Shingen. He always coolly stands by his master Yukimura and aids him whenever possible. He made a rivalry with Kasuga, a ninja in service to Uesugi Kenshin, an enemy of Shingen.
After Toyotomi Hideyoshi died, Shingen fell ill and Yukimura took control over the Takeda army. Sasuke kept aiding Yukimura, even when he had to make terrible decisions. It's often assumed that Sasuke has a crush on Kasuga, but his feelings toward her are closer to that of a brother's.
And he is reprised by Michael Sinterniklaas in the English dub and Takehito Koyasu in the Japanese dub.