From what I understand it’s one of the Wrathful Guardians of Buddhism. If anyone has another explanation please submit it or let me know on twitter!

Edit: From @Hatoriki He has Fudô-Myôô, the Japanese version of Acala, on his back. Here is a Wikipedia article on the deity:

Acala is said to be a powerful deity who protects All the Living by burning away all impediments and defilements, thus aiding them towards enlightenment.

An enigmatic aspect of Buddhist iconography is the presence of wrathful, terrifying forms. Though these awesome, hair-raising images seem contradictory to Buddhist ideals, they are not personifications of evil or demonic forces. Rather they symbolize the violence that is a fundamental reality of the cosmos in general, and of the human mind in particular. In addition to destroying the passions of the mind, the purpose of gods is to protect the faithful. The wrathful deities, who symbolize the tremendous effort it takes to vanquish evil, especially perform this function.