Short answers, pulled from the story.
Seth is the main antagonist and final boss of Street Fighter IV, serving as the CEO of S.I.N., the weapons division of the Shadaloo terrorist organization. The Tanden Engine is a device installed in Seth's abdomen that is central to both his character design and his special moves, including his super combo and ultra combos.
Seth is named after Seth Killian, Capcom's former senior manager. Killian stated in an April 2009 interview that naming the character after him was not his own idea, but that he felt honored as a Street Fighter fan.
Seth was announced as the 40th character added to Street Fighter V: Champion Edition at the conclusion of the 2019 Capcom Cup, with series producer Yoshinori Ono and professional wrestler Kenny Omega appearing on stage together. The full Champion Edition was scheduled for release on the 14th of February, 2020.
In Street Fighter V's story, Seth survived the events of Street Fighter IV by transferring consciousness into "Doll Unit 0", a feminine body type from Bison's Doll Program. The masculine voice remained, which prompted discussion among critics about transgender themes and gender dysphoria in relation to voice and passing.
Seth is revealed to be "Number 15", one of many androids Bison created as potential replacement bodies. Seth rebels against this programming and attempts to overthrow Bison. Another character from the same game, Abel, is revealed to be a prototype of Seth's model.
Reception in Street Fighter IV was largely negative; critics at VideoGamer.com and Eurogamer described him as cheap to fight and a lazy design effort. His return in Street Fighter V received more positive coverage, with writers at Siliconera, The Michigan Daily, and TheGamer praising both his reworked gameplay and his redesign's engagement with themes of identity and self-actualization.