Seto was one of the Shandians who lived on Jaya 400 years ago. He was a curious and brilliant boy with dreams of becoming a great warrior like Calgara, defending his people.
He fell ill during the plague that ravaged the Shandian Tribe, but Montblanc Norland managed to cure him in time. From the beginning, Seto recognized Norland as an honest, wise, and good man. He stood by Norland and his crew, protecting them from the anger of his own people. After Norland's departure, Seto married Calgara's daughter, Musse.
Years later, Seto was seen with Calgara entering the ruins to ring the bell. While Calgara went to ring the bell, Seto cared for Norla, who had made herself at home in the ruins. Suddenly, an earthquake struck, and both Seto and Calgara fled to the village. The Knock-up Stream launched their island into the sky, and Seto found himself living his childhood dream of defending his people when the Skypieans tried to take their land.
Source: http://onepiece.wikia.com/wiki/Seto
