Simon Lewis, at the beginning of the Mortal Instruments series, is 15 years old. He is described as medium height, having brown hair, dark eyes and as wearing glasses. He is part of a Jewish family consisting of his mother, a nurse, and sister Rebecca; his father died when he was 10 years old. He has been best friends with Clary Fray since they were little and has loved her ever since - 10 years in total.
In the City of Bones, Simon turns into a rat after drinking a suspicious liquid at one of Magnus Bane's parties, and is taken to the Hotel Dumort after being mistaken for a vampire's friend. Clary eventually rescues him, but at some point he bites a vampire and injests his blood. The vampire blood draws him repeatedly toward the hotel, and later Simon is Turned into a vampire, though he soon discovers he can go out during the day, because he drank Jace's blood, and the sun doesn't burn him anymore.
Simon resents Jace because of his relationship with Clary, but in later books he becomes Clary's boyfriend. When he realizes that Clary will never feel the same way about him as she does for Jace, he breaks up with her and becomes closer to Isabelle. Then, Simon meets Maia, a werewolf who is part of Luke's pack, and they put aside the fact that they are natural enemies and start dating. Simon is then torn between Isabelle and Maia, who don't know about each other.
When Simon's mother finds out he is a vampire, she doesn't take it lightly. She shuns him from her home and he begins living with "Kyle", another werewolf, although Simon doesn't know this at first. It is later found that Kyle, whose actual name is Jordan, is the boy Maia told Simon about. He Changed her into a werewolf. Despite the fact that his roommate is the reason his girlfriend is a werewolf, Simon becomes close to Jordan.
In the City of Glass, Clary marks Simon with the Mark of Cain, a Mark that causes anyone who tries to hurt Simon to be injured sevenfold, or seven times the damage it would have caused Simon. This proves to be very traumatizing for Simon, watching people being hurt and killed and him not being able to do anything about it, until in the City of Lost Souls, when Simon has his Mark taken by the Angel Raziel.