Musai is a Kumogakure shinobi.

During the Fourth Shinobi World War, he and Nonota were ambushed by a reincarnated Toroi. Recognising his fellow Kumo shinobi, Musai tried to alert Nonota about Toroi's techniques, but they were attacked before he could finish. After Nonota was magnetised, he further explained Toroi's abilities. Both are saved by the arrival of Naruto Uzumaki and Killer B, Naruto slamming a Rasengan against Toroi.

Musai explained how they were headed to Headquarters when they were separated from F and the others, only to be shocked by B, who revealed that F was actually a transformed White Zetsu. In the anime, after they left, Musai and Nonota were briefly suspicious of the other being a transformed White Zetsu as well, before realising the enemy tried to kill them both, meaning they were both on the same side. They laughed it off, and then Musai joked, pretending to be a transformed White Zetsu, only for Nonota to slash his nose with a kunai in defence.

He and Nonota later met up with their regiment while carrying Toroi, after explaining how Naruto and B had saved them, Torune arrived and summoned a group of Kabuto's "failures" to retrieve Toroi and the Third Raikage, among them was Musai's father Osoi, who he noted to a comrade had never made it past chūnin much to his father's annoyance. After promising his father to tell the Raikage he and Toroi had died fighting for the sake of the village, they said goodbye as his father was sealed.

Manga Debut: Chapter #546

Anime Debut: Naruto Shippuuden Episode #296

(Source: Narutopedia)