Jonathan Morgenstern (Sebastian Verlac)
Sebastian Verlac/Jonathan Christopher Morgenstern is the true blood related son of Valentine Morgenstern and Jocelyn Fairchild, making him the brother of Clary Fray. His real name is Jonathan Christopher Morgenstern, but he went by Sebastian Verlac (cousin of the Penhallows) in City of Glass. He is very sadistic in nature, as a result of having been experimented on by his father with demon blood. He first appears in City of Glass as Sebastian Verlac, charming his way into the lives of the Lightwoods. He takes Clary to see Ragnor Fell, and kisses her, disgusting her. Upon Clary's return with Jace to Alicante, it is discovered that Max Lightwood had seen someone climbing the demon warding towers. Sebastian was the person climbing the towers, and it was his blood that was used to bring down the wards. During the first attack on Alicante, he also kills Max Lightwood and severely injures Isabelle Lightwood. He is eventually tracked down and killed by Jace, before the summoning of Raziel by Valentine, but his body is never recovered. He is taken by his "mother" (Lilith), whose blood was used in his experimentation, and taken to safety. She uses Jace as leverage and tries to bring him back from the dead. In City of Fallen Angels he is awoken, and uses a rune put on Jace to control him.
He becomes the main antagonist in City of Lost Souls, using Lilith's bond to alter Jace's memories to believe they are on the same side and plans on raising and army of demons and dark shadowhunters to destroy the world. Clary infiltrates his inner circle through Jace and they both continually mistrust each other but are forced to work together when they are attacked by enemy demons. Eventually Clary learns the truth of his plans and they engage in a bloody battle in with Sebastian almost rapes Clary before she overpowers him. Unable to kill him because it will kill Jace too, she gives up but successfully destroys their base of operations with one of her runes. After a dark ritual, he uses the Infernal Cup to create more evil nephilims and almosts turns Clary but is quickly stopped by the arrival of the Lightwoods and their allies. Before he manages to escape, Clary is forced to stab Jace with the angelic sword Glorious, severing his bond with Sebastian and causing the latter horrible pain. At the end of the book he sends a message to Maryse Lightwood, bloody severed angel wings with a single piece of paper saying ´´I am coming´´.
Sebastian Verlac is the main antagonist of the second half of the Mortal Instrument series.
Read more about this topic: List Of The Mortal Instruments Characters