Matsu was the leader of a group of Solarii worshipers. They believed Mathias would release Himiko and cause the world's suffering to end.
He wanted to use Lara Croft to resurrect Mathias, due to her being his killer. He sent one of his daughters to retrieve one of the relics, d He had his group kidnap Samantha Nishimura, to force Lara to return to Yamatai. Though his plans were cut short due to the intervention of Jonah and Reyes, before he could kill Lara, he was attacked by Sam, who inadvertantly cause Matsu to become the conduit for Mathias' ressurection, though in the process, Matsu's mind was taken over by Mathias, who then killed Matsu's daughters.
Lara then destroyed Matsu's body with a grenade, in order to stop Mathias.