Masaki Kurusu is the last member of those who were known as the Four Gods during the era of VOLTS. He is a resident of Babylon City, the highest levels of Infinite Castle, and is later shown to be a spy from Brain Trust to keep an eye on the lower levels.

His techniques allow him to control light, bypassing any physical defense and inflicting internal damage, thus earning him the title "the Controller of Light" ("Flash Master"). He is also able to use light to blind his opponents' senses.

In the manga, he's Hevn former lover and a good friend of the GetBackers duo. In the anime, he is openly allied with Babylon City; he kidnaps MakubeX on their orders and reveals that he had been spying on Raitei and Volts for Brain Trust while he was one of the Four Kings.

(Source: Wikipedia, edited)