It turned out that Jace was wrong. Izzy wasn't the one to break Simon's Lewis heart.
In reality, Simon Lewis was the one that got away.
For some time Isabelle thought she had finally find somebody suited and tailored for her. Somebody who understands, who truly cares- for who she is, not for her body, but for her soul, for the deepest and most secret part of her nobody sees. It took some time, some Star Wars movies, some sweet dates in restaurants and cafes where she would've never gone otherwise. Some kisses, some promises, some accurate words, some understanding and her ten feet tall and bulletproof walls just collapsed. And she let him in. In everything; in her life, in her house, in her bed, in her heart, in her mind.
For some reasons she was the one to break down and beg him to never leave her and for even more mysterious reasons he was the one to promise her he would never do it. Her- unbreakable, icy Izzy Lightwood started to dream about future, started to hope for a house and marriage, no matter how silly it was, even in her own head. For the first time in her life, she was actually in love and she loved it. Seeing whole world painted in pink and feeling like on top of it. She fell for this stupid vampire; once and for all. Without reason, totally and utterly.
For some time they were actually happy; he looked at her like Jace at Clary and she softened whenever he touched her. They made long conversations and even longer make-out sessions and she let him to drink her blood whenever she noticed he wanted to. They were making double dates with Jace and Clary and watching crappy science fiction movies, which she didn't understand, but she didn't mind- having her around made him happy, so she was around, just like that. Sometimes she even left her armor- her make-up and heels and black leather- in Institute and went out in trainers and jeans.
It all passed.
One day, she woke up in a cold bed, alone, tangled in sheets and found a note- how cliché, sappy, damn sappy- a note saying that he was sorry and he will miss her and she should forgot and live her life. At first she thought it was some kind of a stupid joke, then she was angry- with this flaming anger which ordered her to look for him in whole New York for two months- and then she realized he was really not coming back and she broke down like a tree broken by a hurricane. Devastated.
She refused to leave her room for some time, but it reminded her of Max and how Simon came to comfort her, so she instead stared to wander around Manhattan without any destination. Sometimes she drank until she collapsed on the floor, sometimes she woke up in Central Park without her shoes, sometimes beside a men whom name she didn't know . There was really little her family and friends could do about it. Alec just kept on saying he will broke Simon's neck and Clary was such a reminder of him, that they friendship ended. Jace tried his best to comfort her, but the point was, she didn't want any comfort. Not really. She wanted Simon, Simon, Simon near her, all over her, where he used to be, where he should be.
It took her over a year to recover. Not forget, no, she was sure, she will never forget, never even when she will be old and wrinkled ( but he won't be, he will never be, he will be forever sixteen with his messy hair and stupid t-shirts) but she recovered enough to come back to real life. She started to hunt again, communicate with her family and her reality become, once again, normal, as it was before she fell in love and learnt you can never really fell out of it.
Of course, if you're not vampire. Then, you can fell out rather quickly.
When she turned 22 she married Caleb Whitelaw and she eventually befriended him; he was a good man, strong and honest and delicate with her and he didn't care about her past ( and she was grateful for that). They owned the New York Institute since Jace and Clary moved out to Idris and Alec to Magnus and they've been doing a hard, but good job and their life was as nice and predictable as Shadowhunters' life can be
One night ( she was carring their first child then) she though she saw him. She was looking out of the window and for a moment she would have sworn that she had seen his eyes and the leather jacket she had bought him (once in another life), but he disappeared as quickly as she noticed him.
The second time was when she was taking her children on a walk. It was in Central Park and she kept on thinking: maybe it wasn't him, but it was, he remained the same, he never changed. Only she changed, she grew older every day and every night and she was painfully aware of it.
He truly visited her only once; when she was dying.
She didn't die in battle like Jace and Clary and her husband and her daughter- the youngest one, bright creature with hair as black as night and lips as red as her blood when Isabelle held her in her own arms and begged her to stay ( when she died, why she died she didn't even had a chance to live) . Izzy had a long, nice life and she was dying in her bed from some disease which name she didn't even care to remember. For a moment she thought she was in deliria, because he was standing next to her bed, right there as close that she could've touched him. Still sixteen, still with messy hair and these eyes, these stupid eyes which had captured her once upon a time, in another life.
She decided she doesn't care if he was a ghost or not. Suddenly she realized that even though she was not longer this beautiful girl with black hair and black eyes and broken heart inside her chest, nothing really changed.
He kneeled near her, hold her hand and she died just like that, without saying a word to him and without hearing even one from him, but with a taste of bitter regret (if only if only) on her lips.