Kongoubu Bosatsu is one of the Eight Offering Bodhisattvas, who entertain the Buddhas with an artistic performance, also known as Mai. A diligent and hardworking dancer of the Eight Offering Bodhisattvas who is very earnest in dancing.
Because he's not good at making words of his own emotions, he wears a mask to hide himself. There are several deities who never have seen Mai's face, even among the Eight Offering Bodhisattvas. -- quote from Namu Amida Butsu! Wiki.