Sebastio was a member of the Society of Jesus (Jesuits). In 1600 he was stationed in the village of Anjiro, Japan.
When John Blackthrone first arrived in Anjiro on board the Dutch ship Erasmus Sebastio was there to confront Blackthorne. As Blackthorne was an English protestant working for the protestant Dutch Sebastio was hostile towards him and his crew. Sebastio was quick to inform Japanese samurai Omi - who was in charge of the village of Anjiro - that Blackthorne and his crew were pirates.
After Lord Kasigi Yabu arrived to assess the situation, Sebastio lied to Yabu, telling Yabu that Blackthorne and his crew were pirates. Realizing that Sebastio was lying to Yabu he ripped Sebastio's crucifix from him and pushed it into the ground with his foot. Yabu proceeded to seize the Erasmus and its cargo. Lord Yoshi Toranaga found out about the arrival of the Erasmus and sent for Blackthorne before Yabu could have him killed.
Before Blackthorne departed Anjiro for Osaka, Sebastio was able to obtain Blackthorne's logbook. Sealing it in a large envelope he had the Portuguese pilot Vasco Rodrigues personally deliver it to Father Carlo Dell’Aqua in Osaka.
James Clavell had loosely based Father Sebastio on an unidentified Portuguese Jesuit priest who had confronted the sailor William Adams upon his arrival in Japan.