Rei is a character from the game Nu: Carnival, voiced by Girigi Lee.
Official description from the game’s website:

Appearing as if out of nowhere, this particular clan member has traveled far and wide, picking up a great deal of knowledge along the way. Regularly accompanied by a boreal owl known only as "Father", he marches to the beat of his own drum. He will use any means, good or bad, to achieve his goals, all the while maintaining his own set of beliefs and convictions. For reasons unknown, his essence has a tendency to go haywire, occasionally leaving him in a state of excruciating pain.