Jace or Jase is a masculine given name. It is often short for Jason. It is also, less commonly, a surname.
"What's this?" he demanded, looking from Clary to his companions, as if they might know what she was doing there."It's a girl," Jace said,recovering his composure. "Surely you've seen girls before, Alec. Your sister Isabelle is one."
"And now I’m looking at you,” he said, “and you’re asking me if I still want you, as if I could stop loving you. As if I would want to give up the thing that makes me stronger than anything else ever has. I never dared give much of myself to anyone before – bits of myself to the Lightwoods, to Isabelle and Alec, but it took years to do it – but, Clary, since the first time I saw you, I have belonged to you completely. I still do. If you want me."
"Every time I think I'm missing a piece of me, you give it back."