The story in short
Sett is half vastaya, half human. His father was a pit fighter, and his mother's family was cast out of the vastayan community because of her bond with him. So from childhood Sett was rejected by humans and vastaya alike.
His father abandoned them, so Sett started fighting to support his mother. He fought his way up through the pits of Navori, beat the people who ran them and took over himself. Now he's the Boss, and his mother has made him promise to stay away from the arenas of Noxus.
