He is described in the chapter "Secrets and Lies" as having a lanky frame, dressing in his usual uniform of "old jeans, a flannel shirt, and a bent pair of gold-rimmed spectacles that sat askew on the bridge of his nose."
Luke enters City of Bones as he moves out boxes from the Fray house that Clary's mom wanted to get rid of. Clary knows Luke well enough to realize she can ask him a vague question about what she saw-- or didn't see-- at the Pandemonium Club. She needed answers, and Luke seemed like the person who could give her some insight without tipping off her mom. It seemed like the smart thing to do in her position. Who needs a stressed out, anxious mom worrying about you, right?
What would you do if you saw something nobody else could see?
The tape gun fell out of Luke's hand, and hit the tiled hearth. He knelt to pick it up, not looking at her. "You mean if I were the only witness to a crime, that sort of thing?"
"No. I mean, if there were other people around, but you were the only one who could see something. As if it were invisible to everyone but you."
He hesitated, still kneeling, the dented tape gun gripped in his hand.
"I know it sounds crazy," Clary ventured nervously, "but ..."
Jocelyn comes in, and interrupts the moment between Luke and Clary, leaving just as many questions unanswered as when they started talking.He turned around. His eyes, very blue behind the glasses, rested on her with a look of firm affection. "Clary, you're an artist, like your mother. That means you see the world in ways that other people don't. It's your gift, to see the beauty and the horror in ordinary things. It doesn't make you crazy--just different. There's nothing wrong with being different."
We'll take a look at the rest of the scene tomorrow!