So its 11.30 at night and I am dead tired but this Simon/Isabelle plot bunny hopped into my mind and just refused to leave. So of course, I found my iPad, typed out the entire thing and now I'm sitting in my bedroom on a school night when I should be sleeping.

The things we fan girls do for our fandoms.

I hope you enjoy reading this.

Disclaimer: I do not own the Mortal Instruments.

The Only One

With a final hiss, the last demon explodes into dust.

Simon walks over to pick up the seraph blade from where he had thrown it less than a minute ago, his aim straight and true. He wipes the blood off it and turns to face her.

Isabelle gazes at him thoughtfully, and not for the first time, she marvels at the runes etched on his skin. It took her a while to get used to seeing them. But they're not the only changes he's seen in him over the last three years.

Isabelle remembers how he was cute as a mundane, but now looking at him in Shadowhunter gear, her breath nearly catches in her throat. She saw his transition, from geek Simon to vampire Simon to Shadowhunter Simon, but she still isn't able to fully accept it.

He's the only one who can invoke that reaction in her.

They're both nineteen and he's taller now, so that she only manages to reach his height when she's wearing her favourite thigh high boots. His dark hair falls carelessly into his eyes and she nearly lifts her hand to push it back before he does it himself.

He's retained his vampire looks, with his dark chocolate eyes and messy dark hair, his high cheekbones and chiseled jaw. And she would be lying if she said he isn't attractive. He is. Very, very much so.

Damn, he's caught her staring.

Simon raises a quizzical eyebrow at her and she shakes her head, cleaning off the demon blood on her electrum whip with a flick of her wrist and looping it around her arm in shimmering golden loops. "You okay, Iz?" Simon asks her, his free arm-the one that isn't holding the seraph blade-wrapping around her waist to pull her close. She loves the way he holds her, gentle but not as if she can't protect herself, caring but letting her know that he thought of her as an equal, not someone to be looked after.

He's the only one who can make her feel that way.

"I should ask you that question," Isabelle retorts. "You've been jumpy all day, Simon. What's wrong?" She nudges his chest playfully, feeling the taut muscles beneath his shirt. She feels him tense, and looks up in puzzlement.

"I can't tell you right now, Isabelle but I promise you'll find out later. Just-just trust me, okay?" Izzy's about to protest but his uncharacteristic use of her full name makes her hesitate. Something tells her to go along with him, and so she accedes; an unnatural gesture for her. She's used to fighting her way for everything.

It must be Simon who calmed her down, she realizes. Over the past two years, he's been the voice of reason, the person who calmed her down when she was about to fly off in a fury. Isabelle would never admit it out loud (except maybe to him) but she's glad that she has him. He makes her feel safe. Loved.

Not that they haven't had their fights.

Simon doesn't get angry often; he's often able to deal with her calmly and take everything she throws at him. He's admitted to her that seeing her so passionate and angry was actually rather amusing. (She had let him have it for that one.)

She could barely remember why they had been fighting, but she remembered what had happened after it, all right.

Both of them stood glaring at each other, Isabelle's chest heaving as he ran a hand through his hair, frustrated. "You're so-" Isabelle couldn't even finish her sentence before Simon's lips were upon hers and his hands rested on either side of her head, pressing her against the wall and his body. She reached up and tangled her hands in his hair, and he pressed her right up against him, so close that she could feel every contour of his body.

His arms locked around her hips and she wrapped her legs around his waist, their lips moving furiously. His tongue swiped across her lower lip, asking for entrance and she parted her mouth eagerly. Their tongues battled for dominance while his hands slipped beneath the fabric of her shirt and rested on the bare skin of her back, making her shiver. Suddenly his lips broke contact and his mouth trailed kisses down her jaw, the curve of her neck and settled on her collarbone, the exact spot he had bit her three years ago.

Isabelle bucked against him, and he let out a frustrated groan, his lips claiming hers again. And then somehow they were in bed and his shirt had been flung away on the other side of the room, and he was holding himself above her using his arms, his eyes dark as he looked into her own brown ones.

"Isabelle," Simon breathed desperately. " Tell me to stop now, if you want to...because once i start I don't think I'll be able to stop."

"Don't," Isabelle murmured back. "Don't stop, Simon. Please."

And yet, frustratingly, he still hovered above her, his eyes searching hers. "Are you sure? Izzy, do you really want to-"

"Simon, shut up," Isabelle breathed and then pulled his face down to hers and kissed him.

Oh yes, Isabelle smirks. She loves their fights.

That first night had been complete bliss. And the next day, waking up in his arms...that had been unlike anything she had ever experienced before.

He's the only one to give that to her.

"What are you smiling about?" She hears Simon ask in amusement, his eyes trained on her face as they walked back to the Institute. "I was remembering how cute you looked in that-what was it?" She wrinkles her nose thoughtfully, trying to remember. She couldn't very well tell him that she had been remembering their night of passion; although it certainly wasn't the only one.

"That Halloween outfit!" She exclaims triumphantly. "Holding a sword and everything."

"I swear," Simon huffs, exasperated. "I leave you with my mother for ten minutes and by the time I come back you look like you've been friends all your life. And for the last time, I was five!"

"And with a crown on your head..." She gave him a sideways smile. "Prince Simon."

Simon shook his head. "Isabelle Lightwood, you are the most infuriating woman on the planet."

"But you love me anyways," Isabelle returns. No matter how many times he tells her those three words, she still feels a thrill run through her at hearing them.

"I do."

He;s the only guy who ever tells her that.

She kisses him goodbye and she's turning to leave when he grabs her wrist and pulls her back. "Izzy, come to be rink today at 11.30," he whispers urgently, eyes intent upon hers. "Please."

And then he's gone.

Isabelle's boots crunch on the fallen leaves as she makes her way to ice-skating rink. Simon had been shocked when she told him she had never gone, insisting on taking her. But why had he told her to. She barely hears his footsteps when he comes up behind her and covers her eyes.

"Trust me, Iz," he breathes, and she grumbles. "That's all you've said to me today, Simon. Can't you just explain what this is about?"

She hears him laugh. "It's a surprise."

They come to a hill overlooking the ice rink and then he lets her open her eyes and she sees that he's laid out a picnic for them on the grass and she can't help but laugh. "You dragged me here," she says disbelievingly, "at nearly midnight...to have a picnic?"

"Not just any picnic," Simon says quietly. "Happy birthday, Isabelle."

Her mouth falls open-an uncharacteristic gesture for her. "How did you..."

"I have my sources," he winks. "I thought, for once in your life, you should get to celebrate your birthday. Nephilim don't really celebrate birthdays, do they?"

Isabelle can hardly remember the last time she had a birthday celebration. Mostly she received a few 'happy birthdays' from her family, and presents and that was it.

He's the only one to go to this much effort for her.

"Simon...I-thank you."

He doesn't say anything but he gives her a smile and then they sit on the grass and she lies down on the soft blanket, looking up at the stars above her. Simon lies down besides her and his fingers slip into hers and she clasps his hand tightly. Because right now, she loves him more than she can say.

Suddenly, the sky is lit up by a blaze of colour and she gasps, sitting upright. "Fireworks?" She breathed in disbelief. "At 11.30? How did-"

And then she sees his face and it all becomes clear. He is the one who organized it for her. Its evident in the way his eyes search her face and his lips curve into a smile when he sees the look of awe and happiness on her face.

He's the only one who's ever cared this much.

Overwhelmed, Isabelle flings herself at him and kisses him furiously, trying to convey everything she's feeling into the single kiss. He falls on the blanket with her weight on him and she's lying on his chest when he gently maneuvers himself out from underneath her and pulls her up.

"That was only the first part," Simon says, and there's a nervousness in his eyes. "This is the second."

And then he sinks to one knee in front of her and she realizes what's going on.

"Izzy, the day I met you, I learned that you were a heartbreaker. And after that, I decided you were too much out of my league. I steered clear of you. But something brought me back to you-fate, destiny, whatever you want to call it. And after all those years, I know this: I love you. And I know that I will love you every single day until I die and even in whatever afterlife exists beyond. So Isabelle Sophia Lightwood, will you please do me the tremendous honor of marrying me?"

Isabelle suddenly feels a lump in her throat and the prickling of tears. She never cries but now she feels the strange sensation of a tears trickling down her cheeks. "Yes of course, you idiot," she chokes, half-laughing and half-crying and then he slides the beautiful silver ring on her finger and stands up and wraps her in a hug and she's hugging him back just as tightly, knowing that she's just made the most important but the most right decision of her entire life.

Because he's always the only one for her.

I swear, I am a hardcore Simon/Isabelle shipper. Sizzy ranks right up there on my couples scale with Clace, maybe even higher.

Anyway, hope you enjoyed this one and as always, please review.

Till next time!