"How do you know the chosen ones? No greater love hath a man than he lay down his life for his brother. Not for millions, not for glory, not for fame; for one person, in the dark, where no one will ever know or see. I have been in service to the Vorlons for centuries, looking for you. Diogenes with his lamp, looking for an honest man willing to die for all the wrong reasons."
- – Sebastian
- "A message needed to be sent, etched in blood for all the world to see. A warning."
- – Sebastian
Sebastian was born in England during the 19th century, residing in 14B, Heresford Lane, London. Although his true identity was never discovered on Earth, he was, in fact, the notorious serial killer "Jack the Ripper," who brutally murdered five women in London's East End. Believing himself to have been "chosen", he would later describe his actions as a part of his "holy cause", a warning to a city and a world that he saw as falling into immorality and decay.
By November 11th, 1888, Sebastian's activities had attracted the attention of the Vorlons, who decided to take him into their service. After showing him the error of his ways, the Vorlons took him away from Earth and trained him to be an inquisitor for them – someone to interrogate and confirm that a certain person in the Vorlons' service was the right person acting for the right reasons at the right time. He interrogated many beings over the next few centuries, but none were successful. When not interrogating, Sebastian was put in stasis.
"Good luck to you in your holy cause, Captain Sheridan. May your choices have better results than mine. Remembered not as a messenger, remembered not as a reformer, not as a prophet, not as a hero, not even as Sebastian. Remembered only as Jack."
- – Sebastian
In 2259, Sebastian was summoned by Kosh to Babylon 5, where he was charged with interrogating Ambassador Delenn, to determine if she truly was the right person in the right place for the right reasons. Captain John Sheridan questioned him about how a human came into the Vorlons' service, but Sebastian told him only a little. Sheridan prepared a room for the question and Sebastian waited for Delenn. He had her don manacles that caused pain and focused his interrogation around the Vorlon's question, "Who are you?" He challenged her about her perception of her role in the Vorlons' plan. He almost concluded that she was only following Kosh on the promise of fame or recognition until Sheridan interrupted. Sebastian tied him up and, by hurting him, drew Delenn to declare she would risk her well-being (and the Vorlons' plans) for him. He saw Sheridan would do the same for her. He then was satisfied and stopped the interrogation.
As Sheridan saw Sebastian off, he told him he found out who he was on Earth. Sebastian didn't deny it, but opened up to him. He also expressed hope that he would finally be allowed to die.
It was not known to Sheridan or Delenn what became of Sebastian following his assignment to Babylon 5.
It wasn't clear if the ship Sebastian traveled on was his or simply a transport. Given the later departure of the First Ones from the galaxy, it's possible Sebastian was allowed to die.