Sebastian is described as a focused and confident future agent. His nature is put to test as he meets new challenges through multiple CIA operations. How he evolves and faces confusion with regard to his moral principles and the actions he needs to take in the world of espionage will be a real test.
Sebastian is a former priest who was recruited to the CIA and was trained at the Farm. At the training facility, he shared a dormitory with Harry Doyle, who later became his romantic interest. After graduating from Camp Peary, he becomes a CIA operative. Later, it is revealed that he married, Carly Klapp, a senator's aide.
Attending the G-20 Summit, a year later, it was sabotaged by a terrorist group called the Citizens Liberation Front (the CLF). However, during the hostage escape, it was revealed that Carly was a disguised member of the AIC, a rogue intelligence faction of the CIA, who were planning to initially sabotage the event. She mentioned that she killed a CLF member and threatened to kill Harry if Alex did not turn over Will Olsen, a hacker working for the CLF. It was also revealed that Carly was a former CIA operative who used her marriage as a cover to gain intelligence at the people at the Farm as part of a undercover AIC mission. However, Carly was shot and killed by Sebastian during a gunfight although Sebastian was also shot and taken away by the FBI. He later survived.
Some time later, it was revealed that he had been investigating the conspiracy behind the 2018 Hostage Crisis and tried identifying the people whom Carly had worked for. He later came across Harry Doyle, who was working for the covert task force. Later, he snuck into the Farm and was subdued by both Owen Hall and Ryan Booth. He finally revealed that he was investigating the shadow group that had ties to the people who instigated the hostage crisis. He also revealed the names of the remaining collaborators.
- He is Asian-American.
- He is the fourth LGBT male character in the series.
- Due to his strong beliefs, Sebastian is in denial about his sexuality.
- He can speak and understand Cantonese.
- He is a highly moral and religious person.
- He is a former priest.
- He is a former romantic interest of Harry Doyle.
- He is a former CIA recruit.
- After completing his training at Camp Peary, he becomes a CIA case officer.
- In FALLENORACLE, it was revealed that his wife Carly Klapp used the cover of their marriage as part of an undercover operation with the AIC.
- In KUMONK, it was revealed that, he infiltrated the office at Camp Peary in order to find information that the task force collected so far. Sometime later, he ends up in a short physical fight with Owen Hall. After the fight is over, Sebastian clarified his role in exposing the names of the collaborators. By providing the necessary intelligence to the task force, their mission is considered a success. They expose the following names of the eight collaborators who were directly or indirectly involved in orchestrating the 2018 Hostage Crisis.