Sasano was the owner of Musashi.
Sasano was Musashi's owner. He is very proud of his dog who is a great fighting dog. When Musashi fights and defeats Niōryū, he needs to pull him off of the other dog. He is confused when he later sees a group of wild dogs watching him and Musashi: Gin's group. Not known by him, Musashi runs away and joins the Ōu Army to fight Akakabuto. It seems like that Musashi returned back to him after Akakabuto and Gaia were defeated.
Sasano appears in Tōbē's flashback, during the fight between Musashi and Tōbē. He is annoyed after Musashi lies down and tells him not to let "the brat" win. Likely, Musashi ran away after the battle as he ended his career as a fighting dog.