City of Ashes is Jace's story. He experiences the most drama, the most action (no, not that kind, despite what his sister wants), and the most change out of anyone in the book. From the beginning, Jace is put into an uncomfortable position (again, not the kind you'd find in the Kama Sutra) when he is basically banished from the Institute, forcing him to figure out who he really is, and who he wants to become.
His choices: he could sleep with his sister, but she says she doesn't want it (even though she secretly does), he could turn traitor and join forces with Valentine, his evil father, or he could just curl up in a ball and die.
He does none of the above. By remaining true to himself and his actual family (the family who adopted him, the Lightwoods) he puts a major wrench in Valentine's nefarious plans. Along the way, he discovers he has some sort of magic power: a power to do, well, pretty much anything.
He jumps clean out of a cage and off a building without hurting himself. These superpowers give Jace even more of a pretentious savior complex. Someone even wonders if "Jace can walk on water" (3.17.19). He pretty much becomes Jesus (Jace-us?) so we wouldn't be surprised if he could. Adding to this imagery immediately after, the sun sets behind him, "turning his hair to a halo of gold" (3.17.89).
Jace might think he's the second coming, but he's more of a reverse Jesus. Valentine may be evil, but he's right when he tells Jace,
You like knowing that Alec and Isabelle would die for you. That your sister would. The Inquisitor did die for you […] and you stood by and let her. (3.19.80)
Jesus died for others. For Jace, it's the other way around.
At the beginning of the book, Jace is still a petulant teenager. He starts fights when he doesn't get his way, and he treats Clary like crap when she doesn't want to sleep with him. (Real classy, man.) With Clary, his attitude of choice is to patronize her. "Do you even know how to use that knife, Clarissa?" (2.12.63) he sneers, calling her by her real name.
However, he starts to mellow out as he realizes that the only person he can count on is himself, and no one actually owes him anything. He starts to not hate Simon as much, which is something we never thought would happen, maybe because he realizes Simon is a good guy, and isn't a threat for Clary's affections. Their new friendship is solidified when Jace allows Simon to drink his blood, meaning that Simon gets more action with Jace in this book than Clary does.
Also, Jace actually treats Clary with respect by honoring her wishes:
I'll just be your brother from now on […] That's what you wanted, isn't it? (Epilogue.151)
It's been what she told him all along, and he finally respects that. Sure, Clary's been lying this whole time, but Jace doesn't know that.
Finally, Jace gets accepted back to the Institute by the Maryse Lightwood, who swears she was doing that annoying thing adults do in YA books—pretending not to like Jace in order to protect him. Don't adults know that never works out well? Why are they always so manipulative?
Anyway, Jace ends up in a nice place at the end of the book. Where do you think he'll go next?