Simon is Clary's best friend. He's such a good friend, in fact, that she never feels the need to tell us his last name in this book. With all the name switching going on—Clary's mom and Luke have alter egos, and both Clary and Jace find out they're Morgensterns—we're glad to have one fewer name to keep up with. No last name doesn't mean we don't know anything about him, though. Here's what we do know.
Simon has a great sense of humor. Even when Clary's not paying attention to him (which is a lot), he can be counted on to crack a joke and make us giggle. In fact, within just a few pages, Clary is already ignoring the poor boy, so he tosses out this bon mot: "I've been cross-dressing. Also, I'm sleeping with your mom. I thought you should know" (1.29). Yuk yuk.
Clary even describes Simon as someone who "always says exactly what comes into his head. No filters" (9.19), to which Simon cracks, "Filters are for cigarettes and coffee. […] Two things I could use right now, incidentally" (9.20). Even though we never see him smoke.
Poor Simon gets a bit lost in the shuffle as Clary's adventure with the Shadowhunters starts to take off. Before that happens though, we do get small glimpses into his personal life. He has two older siblings, and a few male friends: Matt, Kirk, and Eric, who writes some really atrocious poetry. They have a band together, although Clary remarks that "Simon's band never actually produced any music. Mostly that sat around Simon's living room, fighting about potential names and band logos" (2.142).
When Simon's not not writing music, he's busy tagging along with Clary and trying to get her to notice him. It's difficult enough because Clary isn't sure if he's gay or not, but when Jace comes into the picture, it gets even harder.
Jace and Simon are almost polar opposites. "[Simon] had a stronger practical streak than almost anyone else [Clary] knew" (8.73), which puts him into direct contrast with spontaneous, romantic, headstrong Jace. Where Jace is described as "the sort of boy who'd come over to your house and burn it down for kicks" (11.230), Simon's "the sort of boy who'd come over to your house to pick you up for a date and be polite to your parents and nice to your pets" (11.230).
Simon also has years of history with Clary, something that Jace definitely doesn't have. Unfortunately, this has led Clary to think of Simon as more of a brother than a lover. Which is ironic, considering that the boy she's in love with turns out to actually be her brother. Oops!
One thing both Jace and Simon have is the uncanny ability to be a smart aleck. In fact, everyone in this book seems to possess that trait. Jace is just as adept with the wisecracks as Simon is, so it's not just your typical brains vs. brawn case.
Simon might be just a little out of his league in the fight for Clary. He ends up with one major advantage, though, which is not being Clary's brother. But when your main pick up line is something along the lines of "Hey, babe, we're not related," you still have a long way to go.
There's a long portion in the middle of the book where Simon gets turned into a rat and has to be rescued. While it does show the lengths Clary will go to for her friend, it doesn't serve much other purpose. (It's not like rat-Simon can utter wisecracks or push the plot forward.) Perhaps it will come more into play in future installments?
Simon's definitely grateful for being rescued, and doesn't seem to be suffering from any prolonged ratty side effects. In fact, it's Clary that acts more like vermin, the way she mistreats his loyalty. With Jace in her sights, Clary practically forgets about Simon. Until she needs a ride to retrieve the Mortal Cup (oh, the trials of saving the world without a driver's license!). Simon dutifully obliges: "He muttered something nearly inaudible, then: 'Fine'" (18.120).
They're lucky they brought Simon as a getaway driver, because the quest to retrieve the Mortal Cup is almost foiled by the demon Abbadon's appearance. Simon's practicality and level-headedness come in handy when he blasts out a skylight with an arrow, raining deadly sunlight down on the evil demon. Hmm, maybe Jace has some competition after all?