Short answers, pulled from the story.
Seth is a synthetic being and the final boss of Street Fighter IV who was introduced in the year 2008. He serves as the chief executive officer of the Shadaloo Intimidation Network and is a prototype android known as Number 15. The character was named after Seth Killian, a senior manager at Capcom, though Killian claimed he never proposed the idea.
The Tanden Engine is a device embedded in Seth's abdomen that serves as the power source for his entire being. This engine allows him to absorb the DNA of other individuals and weave their fighting styles into a single combat style. The engine's power flow becomes visible when his epidermis expands to reveal the inner workings of his mechanical body.
Seth survived the events of Street Fighter IV by transferring his consciousness from his damaged original body into Doll Unit 0. This feminine body type was used in Bison's Doll Program and allowed him to continue his mission with a new physical form. The redesign included a hair knot replacing his bald head and drew inspiration from fierce Buddhist statues like Kongo Rikishi and Nio statues.
In the Japanese version of the game, Seth was voiced by Akio Otsuka, a veteran voice actor known for his work in anime and video games. In the English version, the character was voiced by Michael McConnohie, a legendary voice actor who had provided voices for numerous iconic characters in the Street Fighter series. The choice of voice actors was intended to reflect the character's unique nature as a synthetic being.
The character of Seth was reimagined for Street Fighter V: Champion Edition in the year 2019. The redesign included a hair knot replacing the character's bald head and drew inspiration from fierce Buddhist statues like Kongo Rikishi and Nio statues. The developers also explored the idea of Seth's arms and legs transforming in the manner seen in Steampunk-themed movies and books.