This is how it should start.

There's a boy and there's a girl.

In theory, it's simple; black and white, and white and black, with a couple of specks of dark red thrown in between, but it's still simple - and that's how it should be.

(In reality, it's complicated, and it was from the start, and it will be until the end.)

.

Her head is in his lap, and she's wearing nothing but one of his old t-shirts and her underwear. He can feel her warmth seeping into his cold skin, her breath on his old blue jeans. He catches himself running a hand through her dark hair, and quickly stops himself, moving her slightly so her head is resting on a pillow rather than his lap. He sees her eyes drift open slowly, and quick as a flash, he's off the bed, just leaning against it.

"Hey," he says gently, Isabelle's eyes still half-closed. "I should, um, probably go now." Her eyes open up more at his words, and she turns her head to the side so she's looking at him directly.

"You can stay, if you like," she says quietly.

He just shakes his head.

"Bye, Izzy," he says.

.

On Clary's seventeenth birthday, the sky explodes with fireworks of every colour. Red, yellow, green, pink, purple, blue; a complete rainbow fading in the sky. They celebrate her birthday in a park, the skyscrapers surrounding them, with streetlights giving little cracks of light among the shadows.

There's a bonfire burning as well, everybody sitting around it and laughing and drinking. He watches as Clary laughs with Jace, her hair the same colour as the fire, lighting up her face. He feels movement next to him, and turns to see dark hair and dark eyes, the fire reflected in them.

They don't speak, just look into the flames and feel content with the feeling of each other close by. Slowly, Isabelle reaches for his hand, lacing her fingers through his and gripping them tightly. A moment later, when his hand only just grips hers back, she lets go.

.

(This is where it gets complicated.)

.

Simon has been sixteen for two years when he runs away.

He just hops on a plane and goes, not bothering pack a bag or check where his flight is going to. He leaves without saying goodbye to anybody, and doesn't even bother to leave a note (but he leaves a Bible on his bed, and he figures that's clear enough as anything else).

He sends three postcards: one to his mother, one to Clary, and one to Isabelle. He ends up sending them all blank, because really, he doesn't have anything else to say.

.

He travels in the day and sleeps in almost broke hotel rooms at night - and this really, really, isn't how it should be, he knows.

He doesn't do much; there's not really much to do. But he writes letters over and over, and never sends them, sits in the sun, and tries to get the words stuck in his throat off his lips.

(Yahweh.

Allah.

Jehovah.

Dios.

- God, why can't I say it?)

.

He should be twenty when he comes back, and he is, but he's also sixteen.

He's walking around Brooklyn, his hands in his pockets, scuffing the same black and white converse he's had for four years now. The streetlamps light his way, making his skin seem almost translucent. Suddenly, he hears footsteps against the pavement behind him, and turns around to see Isabelle.

"Simon?" She asks cautiously, staring into his shadow. "It's you, isn't it?"

"Ugh, yeah?" He replies, rubbing the back of his neck nervously. He sees her start to walk closer to him, slowly, as if he might runaway (he will, but not yet). She comes up until they're looking straight in each other's eyes, and, somehow, he finds himself almost shocked at the tiny, almost unnoticeable signs, that she's gotten older. She doesn't hug him, just stands there and looks at him.

He shakes his head, blinking the image away. Pretending to cough, his gaze moves to his shoes. Glancing back up for a moment, he says, "Sorry, having a weird flashback from when I was sixteen," he laughs internally. She doesn't smile though. He sighs.

"I'm go -" She grabs onto the sleeve of his shirt, turning him around as he prepares to walk away.

"I want you to stay," she says quietly. He smiles at her, a little sadly, though he hopes she doesn't notice, and gently pries her fingers off his shirt.

"I know," he says. "But you don't need me to stay."

He sees her eyes dim as soon as the words come out of his mouth, and immediately, her eyes have a reserved, shielded look about them. She crosses her arms over her chest, not quite being able to look him in the eye. He reaches a hand up to touch her arm, but it falls short, dropping back to his side, skimming across her skin for less than a second.

"Bye, Izzy," he says, almost whispering.

And then he's gone.

.

And this is where -

he doesn't know anymore.

.

He falls back into the same pattern, going from place to place and continuously missing where he really wants to go (home home home).

(Yahweh, Allah, Jehovah, Dios, God. He just gives up on it all.)

.

He goes back for Clary's wedding when she's twenty two (it's easier to count their time rather than his, and us far less pointless). He watches them from outside the church through the coloured window, just able to make out smiling faces and glassy eyes, and Clary's forever vibrant red hair.

He moves aside when he sees everybody begin to move, hears the creak of the old wooden doors as they open. He's just about to leave when Clary catches sight of him, and before he knows it, he's been enveloped in a hug, small tears beginning to stain his shirt.

He wraps his arms around her, holding her loosely compared to her grip around his waist (will you stay? no.) He ignores the quick, shy stares that pass his way, but he instantly lets go when Jace comes towards him, giving him a quick nod in reply.

He leaves before anyone else can grab onto him.

.

Except he can't - not yet.

So he manages to find Isabelle's place and climbs up to her window sill, opening the window and sliding himself in. At the sound of the window sliding up, he sees Isabelle look up from her bed, a wary look on her face that softens ever so slightly when she sees him. He sits on the window sill, smiling at her.

"Of course, you come back for Clary," she says simply, looking down at her patterned quilt. His smile fades slightly, and his grip tightens on the thinning wood, his body preparing to runaway.

"But I stayed for you," he says. At this, all harshness disappears from her face, and she comes over to him, and he thinks he sees the beginning of tears welling in her eyes.

"Are you going to leave again?" She asks, though he thinks they both know the answer. Without a word, he nods his head. you'll never stop, they both think. She brings a hand up to his face, and lightly brushes his fringe away to reveal the white mark on his forehead.

"Is this why you left?" She asks. He nods his head again slightly, not quite being able to form the words in his head - he begins to shake his head.

"It's complicated," is all he says.

"I just don't get why -" her voice begins to get louder, less controlled.

"I left because of you," he says softly. She stops, her face blanches white, and he suddenly feels tired, older, and isn't it just so fucking ironic?

She opens her mouth, but before she can say anything he leans over and kisses her (because hell, somehow, he's always been more of a romantic than she is), and it's only then that she notices the small, sparkling ring still in its velvet box on her bedside table. Instantly, he closes his eyes.

She's the first one to break apart, one tear managing to stain her cheek, which she quickly wipes away.

It only takes one glance for him to get the message, and then he's out the window again.

.

You know the story by now.

.

He's been in far too many hotel rooms, so many that he's lost count by now. But they all look the same and have the same purpose, and all that changes is the view from his grimy window. He doesn't know where he's going anymore, though he's not sure he ever did, but now he's not just wandering - he's lost.

It's been far too long now, he realises. He looks half his age and feels double it, and nothing about him has changed, yet everyone around him is slowly decaying. Suddenly, he's filled with anger and sends his fist flying through the window, the glass shattering with an impressive sound as it crashes to the ground. His palm is still intact, without one single speck of blood on it.

It doesn't make him feel any better.

(Yahweh, he whispers, the name seeming to sear his throat as he says it. He looks down at the cross temporarily imprinted onto his palm.

time changes, he thinks bitterly.)

.

Isabelle finds him. He doesn't know how; he doesn't bother to ask, but somehow, she finds him.

She welcomes herself into his latest hotel room, sitting herself on the bed that he barely ever uses. He can see a gold ring glinting in the flickering fluorescent light. She gives him a smile, but he can see wrinkles deepening themselves at the corner of her mouth, and that it doesn't reach her eyes anyway.

"You know," she starts straight away. "It wasn't your place to judge. I was an adult, I could've chosen for myself." He goes to sit by her side on the bed, smiling to himself a little at the re-surfacing memories that he'll regret once she's gone.

"You came here to tell me that?" He asks. She ignores him.

"And, just so you know, I would've been strong enough to take it. I still am," she says. His smile drops.

"Yeah," he says. "But I wasn't. And I'm still not."

They're silent for a few heartbeats, the clock that hangs in the hotel still ticking away loudly, even though it's an hour and a half behind. Without him noticing, Isabelle manages to put her fingers lightly over his, and even when he does notice, he doesn't move away.

"You were always going to lose me," she says, a slight hitch in her voice. "Immortality, Mark of Cain, either way…" she drifts off.

"I didn't want to lose you sooner than I already had to," he says. "And I didn't want it to be my fault."

She doesn't really have an answer for that.

.

They stay by each other's sides for more than an hour, just resting on the bed and listening to each other's breathing and the sound of the clock. Just being next to one another. Finally, Isabelle stands up, plants a kiss on Simon's cheek, and begins to walk towards the door, before stopping and turning towards him, her hand resting gently on the handle.

"I'll see you soon," he says, wondering if it will be true or not. She gives him a small smile, which this time reaches her eyes, and suddenly, she looks younger - and a little bit sadder - than she did before.

"Bye, Simon," she says.

.

This is how it ends: with a boy and a girl and an ever after.

It's not simple.

.


Disclaimer: I do not own the Mortal Instruments. Title goes to Teenage Crime - Adrian Lux.


A/N: First time writing for this fandom. I wanted to focus on the mark of cain, as well as his immortality, so I hope that was all ok. Also, just for interest, Simon would probably have to leave, merely because that's sort of what's in the deal of having the mark of cain - you wander, and you're cursed.

So now you are cursed from the earth // A fugitive and a vagabond you shall be on the earth.

PM/Review me if you have any questions, hated it, liked it, etc.