Sebastian came from a privileged family. He was captured by Scion for unknown reasons and sent to the penal colony along with Paige and her fellow voyants. He was placed under Graffias Sheratan's control.
Seb's spirit was discovered to be the guardian of the padlocked door leading to the train station when Paige and everyone in Sheol I were escaping. Paige was able to convince him to open the door, and shortly after she released his spirit by speaking the threnody.
Sebastian is a Latin name that means "from Sebaste," which is derived from the Greek word for "venerable." Pearce is a derivative of Piers, an Old French and Middle English form of Peter (rock).