The main antagonist of Street Fighter IV. He is the head of Shadaloo's weapon division, S.I.N. According to the official Street Fighter website, Seth is one of many genetically engineered replacement bodies for M. Bison.
Seth returns in Street Fighter V as Female Seth. With his body destroyed at the end of Street Fighter IV, his surviving brain is combined with the body of a Shadaloo Doll named Unit Zero, resulting in Seth adapting a new female appearance.
