(This section contains spoilers. If you haven't read any book other than "City of Bones", you might want to stay away from here. Don't say I didn't warn you. :P)

Simon Lewis is Clary Fray's best friend. He accompanies her in her adventures, despite being human for most of the first book. In “City of Bones”, he is kidnapped by vampires after being turned into a rat and brought to their nest in the abandoned Hotel Dumort. Later, in “City of Ashes”, he begins to feel some side effects as a result of biting Raphael, the temporary head vampire while he was a rat. Because he is human and doesn't know the correct way to become a vampire, he begins to fear he may be turning into a monster.

When he cannot reconcile his feelings for Clary, he returns to the hotel—putting himself in great danger—to ask them if he is becoming one of them. Unfortunately the vampires attack, and Raphael saves him from immediate death by bringing him back to the Institute, where Clary is staying. Faced with the choice of either letting Simon die or making him become a vampire, Clary and Jace decide to let him be reborn as a vampire. In “City of Ashes”, he becomes a "Daylighter" after being drained of most of his blood on Valentine's ship by Valentine so he can complete his Conversion on Maellartach, The Soul-Sword. Jace finds Simon on the brink of death and allows him to drink his blood to revive him. The large quantity of Angel blood in Jace's body then allows Simon to stand in high sun with no risk of burning, as all other vampires do.

After Max Lightwood's death, Isabelle (Max's older sister) becomes distraught, blaming herself for not listening to him and for not being there for her little brother. She refuses to see anyone, finally allowing Simon to come in. Simon comforts Isabelle, trying to convince her that Max's death wasn't her fault, and eventually ends up sleeping next to her, much to his surprise, although they did nothing sexual. Towards the end of the third novel, Raphael demands Simon's death in exchange for his clan's assistance for the war; Raphael believes Simon's Daylighter powers make him too dangerous to remain alive. In order to protect him, Clary draws the Mark of Cain on Simon, which prevents him from being harmed by anyone and also curses him, marking him as a wanderer for eternity.

In “City of Fallen Angels”, a conflicted Simon tries casually dating both Isabelle and Maia. When the girls find out, they both leave him, and he is alone once more. However, by the end of the book it is clear that a deepening relationship is forming between Simon and Isabelle.

Simon appears in all books of the series: “City of Bones”, “City of Ashes”, “City of Glass” and “City of Fallen Angels”. The fifth book, “City of Lost Souls” comes out on May 8th, 2012.

Source: Wikipedia.