Musha is Black's male Munna. Musha has been with Black since he was five years old. Musha and Black's other first Pokémon, Brav, seem close, enough that when Black lends Brav to White, he wonders if Musha is a little lonely.
PastMusha, presumably attracted by the power of Black's dreams, followed the five-year-olds Black, Bianca, and Cheren as they rashly left Nuvema Town. When Bianca was attacked by a Rufflet and Cheren ran to help her, Musha rushed at Black and ate his dreams for the first time.
Musha later agrees to help Black capture the Rufflet in exchange for eating more of Black's dreams, and then again agrees to join Black permanently for access to his dreams.
Musha debuts in BW Chapter 2.
Black sends Musha out against N but is losing badly, even after having Musha eat his dreams to help him focus. Musha fights his Purrloin. Musha eats N's dream and discovered that N's dream was a gentle one of childhood.
While making their vows in Striaton City, Musha suddenly leaves happily, sensing a powerful dream nearby to eat. He goes to the Dreamyard, which has been with the dream of creating something, where he is taken prisoner by Team Plasma. He manages to free himself after Black arrives and thinks of his dream, the stronger dream drawing Musha back to him. Musha then defeats Team Plasma's Watchog.