Seto was one of the Shandians whom used to live on Jaya 400 years ago. He was a curious and brilliant boy. He wished to be a great warrior like Calgara and defending his people.
He was also affected by the plague that has ravaged the Shandian Tribe. But Montblanc Norland manage to cure it in time. He knew from the start that Norland was a honest, wise and good man and he defended Norland and his crew from the wrath of his people. At some point after Norland's departure, he married Calgara's daughter Musse.
Years later, he is seen with Calgara entering the ruins to ring the bell. While Calgara went to ring the bell, Seto fed Norla, who had made herself at home n the ruins. Suddenly there was an Earthquake, he and Calgara fled to the village. The Knock-up stream shot their island into the sky, Seto found himself living his childhood dream to defend their people, when the Skypieans had come to take their land away from them.

Source: http://onepiece.wikia.com/wiki/Seto