Sebastian Verlac was a Shadowhunter and a cousin of Aline Penhallow. He was raised by his aunt Élodie Verlac who took over as head of the Paris Institute after his parents' deaths. Some time in 2007, he was killed by Jonathan Morgenstern so he could take over his identity.
After his uncle Patrick moved to Beijing, Sebastian's mother, Patrick's younger sister, was sent to Paris, where Sebastian's parents supposedly met. While raiding a demon nest near Calais, France, Sebastian's parents were both killed. Sebastian was then taken in and raised by his aunt Élodie.
Around 2007, seventeen-year-old Sebastian was on his way to Idris to represent the Paris Institute at the emergency Council meeting about Valentine Morgenstern. He was sitting at a bar in a club in France when he was approached by Jonathan Morgenstern. Initially skeptical, Sebastian relaxed in the boy's presence when he learned that he was another Shadowhunter. The white-haired boy convincingly introduced himself as Andrew Blackthorn of the Los Angeles Institute who was on his way to visit Alicante.
They then decided to hunt and kill the Eluthied demon rumored to have been lurking around the place. Jonathan soon spotted the demon in the shape of a girl, so the pair followed the demon to an alley outside the club. After witnessing Jonathan viciously kill the demon, Sebastian was impressed, not afraid as Jonathan had expected. When Jonathan told him that he needed to leave, Sebastian was disappointed. Jonathan then proceeded to slit Sebastian's throat, catching the boy by surprise. Jonathan, not wanting Sebastian's body to be discovered, dumped Sebastian into a nearby river, where Jonathan thought he may have seen Sebastian's fingers bob in the current before sinking below the surface. Jonathan then went to the hotel Sebastian was staying to retrieve his belongings and soon dyed his hair black to completely mimic him.
Tricking Sebastian and taking his identity was easy for Jonathan; not only had Sebastian not seen another Shadowhunter in quite a long time, but even his relatives in Alicante were no longer familiar with his appearance, so the Penhallows welcomed him into their home without question.
When Clary Fray was shown a picture of him by his cousin Aline Penhallow, she described him as handsome in an off-kilter sort of way. He was said to have a large nose, black hair, and a crooked grin. She also stated that Sebastian seemed to have been a person with whom one could have a good time.
Jonathan also described Sebastian as tall, about the same height as him, and with the same slender build. His eyes were black, and his features, though irregular, came together pleasingly: he had a friendly charisma that was attractive. He looked like it was easy for him to smile and trust.
Sebastian is a masculine name for German, English, Swedish, Norwegian, Danish, Polish, Finnish, Romanian usage. It was derived from the Latin name Sebastianus which means "from Sebaste", which is the name a town in Asia Minor, its name in turn derived from Greek σεβαστος (sebastos) "venerable" (a translation of Latin Augustus, the title of the Roman emperors).