Souta was a member of the Ghoul Restaurant, where he went by the alias PG. He hides his face with a clown mask.

It has been revealed that he is from the Pierrot before being a member of the Ghoul Restaurant.
He was revealed to be the one who caused the "accident" that led to Rize's kakuhou being implanted in Kaneki.
(Source: Tokyo Ghoul Wikia)