Musei Shinrai

is a Ninja of Kumogakure. He was friends with the Eight-Tails jinchūriki and served on the Eight-Tails Protection Squad, a unit formed to keep Tenka Nihisaki out of trouble, defend her if attacked, and assist in subduing her if the Eight-Tails ever broke free and ran rampant.

His relationship with Tenka went from friendship to romance as they became closer to one another over the years. He soon became the only person she confided in, as well as the only person that could keep her in check. Together they created the Fire Release: Flame Blade technique, he even created his own off shoot known as the Thunder God Palm. When Tenka betrayed Kumogakure, attempting to flee the Land of Lightning, Musei was among those sent in the initial effort to turn her back. While faltering, Tenka mistook Musei's movement as a possible attack and, on reflex, pierced straight through his stomach in response. While she was shocked by what she had done, the murder of her lover forced Tenka's resolve into abandoning Kumogakure completely.