Gengetsu Hōzuki

He is the Second Mizukage of Kirigakure.

Background
The Second Mizukage was killed by the Second Tsuchikage. However, before his death he was able to kill the Tsuchikage too.

Shinobi World War Arc
Mū, as a former Kage, is resurrected alongside various other notable shinobi by Kabuto Yakushi, in preparation for the Fourth Great Shinobi War. He, the Third Kazekage, and the Fourth Kazekage are sent to provide backup for the Second Tsuchikage in fighting the Fourth Division. Although he wants to try and kill the Tsuchikage again, he is forced to join the others in battle.