He awoke to the rising of the sun.
In the dim light he could only see the outlines of the things in her room, but that was more than enough for him. He looked over at the beautiful figure lying beside him. She was sleeping on her side, facing away from him. In the early morning light he could just barely make out the fiery red color of her hair, which was strewn over her pillow. He could see her chest slowly rise and fall as she slept. She curled up into a ball on her side, shivering. He pulled the blankets, which must had been kicked to her ankles during the night, to her shoulder. She stopped shivering and rolled over to her other side so that she was facing him.
He realized how much more beautiful she looked in her sleep. All of the muscles in her face relaxed, him a facial expression without the fear and worry that he had seen on her delicate features for what seemed like the longest amount of time. It still didn't seem fair to him that the universe had played such a cruel trick on him. Giving him such a stunning goddess and then snatching her away again was like... he couldn't even think of a comparison. It was the cruelest thing that had happened to him during his short and miserable life, but meeting her was also the best and most wondrous thing that had happened to him. She was his light in the dark, his compass in the sea. He had no qualms about giving up his own life so that something so precious could live.
"Jace," she mumbled. He could see her eyes moving beneath her eyelids. She sighed in her sleep. "I love you, Jace."
And that was what broke him. Not the intense pain of the many injuries that he had suffered at the hands (or claws) of various demons, not the insults from his opponents. Not his troubled childhood. Not even learning that fact that she and he were siblings. It was the fact that he was indisputably in love with her, and she with him, but nothing would ever come of it. Nothing ever could come of it. He felt a tear run down his cheek, and he saw it land on her bare shoulder. She didn't even seem to recognize that a tear from the only person she loved had landed on her uncovered skin. He reached across and put his hand in the curve of her waist, and they lay there for what seemed like several lifetimes for him. Several wonderful, beautiful lifetimes.
The sun was what finally drove him out of bed. People would be wondering where he was, and he couldn't be caught leaving her room. He sat up and swung his legs over the side of her bed and into his boots, which he then proceeded to lace up. He buckled up his weapons belt, and then he quietly rummaged through an open drawer for a piece of paper and a pen. He took his time about what his last words to Clary would be; he wanted her to remember him not for his attitude or for his looks but for him. He took off his ring and laid it along with the note on the bedside table. He then grabbed his jacket and was about to leave, but then...there she was. Because of the increased amount of light in the room, he could clearly see that she was breathtaking. Her hair glimmered in the early morning light, and her delicate features looked as inviting as ever. He couldn't turn back. He had to save her, and so he left.