Short answers, pulled from the story.
Capcom senior manager Seth Killian was honored to be part of the series' history, but series producer Yoshinori Ono introduced the new antagonist during a Eurogamer promotion. The character named after him was not his own idea.
Seth is revealed to be Number 15 among many androids created by M. Bison intended as replacement bodies for the terrorist leader. He rebels against his programming to pursue his own agenda and eventually transfers his consciousness into Doll Unit 0.
Ryota Niitsuma served as assistant producer for the arcade version of Street Fighter IV and acknowledged that Seth was overpowered following the launch of the game. Updates would later tweak him to be more balanced.
His body has been heavily modified using advanced technology to become more than human and he collects data on the world's best fighters to borrow their moves. Players can steal special moves from all 40 characters in the roster including stealing a move from another Seth if one exists in the match.
VideoGamer.com writer Wesley Yin-Poole described the character as cheap to fight against while Simon Parker from Eurogamer disapproved of the near-unblockable Ultra move. Gavin Jasper of Den of Geek stated Seth was easily the lamest of the series' villains and felt the character was inferior to M. Bison, Gill, and Sagat.