I want to thank aimmyarrowshigh for helping me get this story rolling, offering creative feedback and doing some amazing beta work. Check out her Mortal Instruments stuff on LiveJournal because it's awesome. Snapcrakklepop was kind enough to edit the first couple of chapters, and I never would have gotten to chapter three without her. Thank you to pnai_87 for talking narrative, characterization, and story structure with me. Her insight was priceless. And thankthatstar gets lots of hugs for being the most supportive Jace in the world.

Warnings: Mature themes including perceived incest, explicit sexual content, and strong language. Immature themes including excessive dash usage and copious amounts of unapologetic relationship angst. Proceed with caution.

Context: This story is set after a City of Glass in which Clary and Jace do NOT figure out that they aren't really brother and sister. They never got the Book of the White to Magnus, so Jocelyn is still comatose. (This ignores City of Fallen Angels entirely.)

Keep the Next Breath

PART ONE

They leave for Idris as estranged brother and sister; they come back as two battered parts of the same whole.

When Clary closes her eyes, she sees Jace empty, pale, gone. He's a shadow on the ground that slips through her fingers whenever she reaches for him. There is no angel. No wish. No what you want most in the world. Every night is this reoccurring Hell she's been delegated to, and it doesn't take much to figure out why.

When Jace closes his eyes, he sees his father. He's standing at the edge of Lake Lynn, as large and formidable as he had appeared to Jace as a young child. But it's the sword in Valentine's hand that terrifies him. It's the tip pressed to Clary's neck, drawing a trickle of blood that Jace feels coating his own hands. Valentine turns to him, spits the word sister like a curse, and then allows Maellartach to claim the rest of her.

[ - ] [ - ] [ - ]

"This is it."

Clary steps around Jace into the small room. It looks just like his, like all of the bedrooms in the institute—four stone walls, twin bed, crisp white sheets, small nightstand, bathroom, closet. It's exactly what she expected. It's what she wanted.

So she doesn't know why she's overcome with the urge to turn and run. Run back to the hospital, where she can sit holding Jocelyn's hand and pretend that she'll wake up at any moment. Clary's not accustomed to running away, has never wanted to this badly.

She forces herself forward until she can drop her overstuffed bag on the bed. She'll need to make another trip to Luke's to get the rest of her things. There isn't a lot—most everything was lost with her and Jocelyn's apartment—but without Luke being here to drive her, she has to make multiple trips on the subway.

She turns and looks at Jace. He's leaning in the doorway, hands stuck in the pockets of his dark jeans, grey t-shirt clinging to his chest. Somehow he looks different than he did in Idris, more real almost. Everything that happened there had had a dreamlike quality to it, and returning to New York has been like waking up from a beautiful, but haunting, nightmare.

"My guess is dinner will be around seven," Jace says offhandedly. "And you don't have to worry about Isabelle being allowed anywhere near the kitchen. Maryse has been cooking every night since we got back. It's therapeutic or something."

His golden eyes follow her progress as she crosses the room, and when she stops right in front of him, she sees his throat working as if he's swallowing something painful. She just manages to keep herself from shivering at his proximity. The heat from his body is an almost tangible thing between them as she peers up at him. "Jace…"

"Something wrong with the room? I know the view of the pothole-riddled street is sort of obscured by the broken street lamp but—"

"Jace." She reaches for his hand, and he lets her take it. Her fingers slide easily between his, and she's struck by how mismatched they look—her pale, freckle-dusted knuckles alongside his smooth golden ones. To think that the same blood runs through their veins…

Jace's free hand brushes her cheek with the hesitancy of someone doing something they know they shouldn't. "I'm glad that you're here," he says, fingers following the curve of her ear. "I hadn't heard from you in a couple of days."

"I know. I've been at the hospital a lot. And then I was at Luke's getting things together. Sorry I didn't call."

He doesn't ask about Jocelyn. Not because he doesn't care but because he knows Clary doesn't want to talk about it more than she has to. The fact that he—that anyone here—knows her so well is a small comfort that pools like warmth in her stomach.

She tightens her hold on his hand. "Jace, we never talked about Idris. We never talked about that night…"

His hand drops from her face. "Clary." He says her name like a warning.

But his letter—the one she's read over until she no longer needs to read in order to hear the words in her mind—is burning a hole in her back pocket. "You meant it. Didn't you? The things that you wrote—"

"Of course I meant it," he says abruptly, and for a moment she thinks that he might wrench his hand from her, but he doesn't. "I can't tell you 'I love you' and not mean it. And I can't say it and mean it the way I should."

"Then why are we—"

"You know why."

"I'm part monster. Part everything I've tried so hard to burn out, to destroy…It explains why I feel the way I do about you…I should want to protect you from the sort of boys who want to do to you exactly what I want to do to you."

Jace's cold, resigned face from that night in Idris flashes through her mind's eye, and she's gripped with the same desperate need to make him see just how wrong he is. Demon blood or not, he isn't a monster. He's one of the most loyal and compassionate people she's ever met, and he's never used his skill for evil, even though he had every opportunity to.

She wants to tell him all of these things, but he draws away before she gets the chance.

"I'm next door," he mutters while turning to leave, "if you need anything."

[ - ] [ - ] [ - ]

There's something dark inside of him. Jace feels it twisting his stomach and sharpening his senses every time Clary walks into a room. His eyes always fall on her hair first. He recalls how the red curls felt tangled around his fingers as he held her mouth firmly against his, and he doesn't even feel guilty.

"Sister" has stopped meaning "sister" and has started meaning "the girl he's in love with but can't have." It's like being stuck in the first stage of a relationship—boy sees girl, boy likes girl—and not having the chance to go any further.

Clary doesn't think there's anything wrong with him. Maybe it's because she's so good. Pure. Maybe it's the Angel blood blinding her to his darkness. He told her in that letter just what she meant to him—she means everything. He knows that he revealed too much, that it was selfish of him to put his needs above her own wellbeing and admit he loves her more than he's ever loved anything or anyone, more than a brother should love a sister. But he'd been so certain that he would never have to deal with the repercussions. When Jace left Clary sleeping in the early morning, he had thought that was the last morning he would ever see.

And it should have been. He died, and Clary had brought him back. If she knew…

The door to the training room opens, and Clary enters with Robert just a step behind her. She's laughing at something that causes Robert to shake his head. Neither of them notices him leaning against the far wall as they cross to the weapons rack, but Jace's golden eyes dog Clary's every step.

If she knew that when he wasn't having nightmares about her death, he was dreaming about her moaning his name as he takes her against the wall…he thinks she would finally understand just how dangerous he is.

[ - ] [ - ] [ - ]

"What's up?"

Clary quickly flips her phone closed and tries to hide it beneath her thigh. She meets Simon's curious gaze evenly. Every once in a while she still does a double take when she sees him without his glasses. His eyes are so much bigger and darker without them. "Nothing," she says too quickly.

He nudges her with his elbow. "Spit it out, Fray."

"It's…" Clary glances at Isabelle, who's sprawled on Simon's living room floor, a bowl of popcorn next to her head and eyes glued to TV screen, where Jensen Ackles is throwing his weight around in some dumpy bar. When Clary had casually mentioned that she was going to visit Simon, Isabelle had insisted on tagging along. Clary doesn't really mind the company—and she knows Simon doesn't mind—but it's strange having a third person around.

She scoots nearer to Simon on the couch so that she can lower her voice. "It's Jace."

They're too close for Simon to turn his head, but Clary can hear the frown in his voice when he says, "He's supposed to call you?"

"I asked him to. I wanted to maybe get dinner later." She sighs in resignation. "We hardly ever see each other."

"But you live together."

"He's avoiding me." He's been avoiding everyone since the day she moved into the Institute nearly two weeks ago. Isabelle writes it off as his way of grieving, and Alec is too preoccupied with Magnus to really notice; it's like the only person who cares is Clary.

Simon takes a moment to respond, and when he does, he speaks slowly. "Maybe that's a good thing. Maybe that's what he needs to do."

Clary frowns. "What's that supposed to mean? We're…family. We're supposed to stick together."

"You're supposed to stick together and not get along," Simon corrects her. "Trust me. I've been condemned to siblinghood a lot longer than you have."

Clary can't even begin considering the differences between her relationship with Jace and Simon's relationship with Rebecca. "Something's bothering him," she says. "I can tell."

She knows exactly what's bothering him. But Jace hasn't told anyone about what they discovered in the basement of the Wayland's manor house, and it's not her place to say anything.

She startles at the feel of Simon's fingers on her face. He's frowning while gently tracing the skin beneath her eyes. "Something's bothering you, too. You look like you haven't been sleeping."

"Hey, what are you guys whispering about?" Isabelle pops up on her knees in front of them, and Simon's hand drops from Clary's face. The television show is momentarily forgotten in the background, as Isabelle's eyes dart between Clary and Simon, lingering where their thighs are touching.

"Nothing." Simon stands and heads towards the kitchen. "Just discussing the weather."

Immediately disinterested, Isabelle turns back to the TV. "All this freakish snow, right? I've already ruined a pair of boots."

[ - ] [ - ] [ - ]

Three days later and Clary still can't catch five minutes alone with Jace. By the time she wakes up, he's already at work in the training room, and when she gets back from her mornings at the hospital, he's out doing no one knows what in the city (at least, Isabelle and Alec certainly can't tell her). She knows he's avoiding her, and she would be hurt by it if she didn't have a good idea as to why.

Clary shakes her head to try to clear away the excess thoughts. She's supposed to be focusing. Twenty minutes in the training room and she's only managed to make it successfully through the set once. Robert Lightwood has spent the last couple of days teaching her some very basic stances and movements that he explained were the basis for any solid defense or offense. At first Clary had been surprised by his volunteering to take her under his wing. Him approaching her and offering to help with her training was the first conversation they ever had. But the more time Clary spends at the Institute, the more often she finds Robert in the training room. If cooking is Maryse's way of dealing with her grief, then physical exertion is Robert's.

Squaring her shoulders, she bends her knees until she finds her balance. Then she begins the set. It doesn't come naturally, and she moves slowly at first, her arms stiff and robotic as they sweep from one side to the other. So she closes her eyes and tries to focus on how it feels instead of how it looks. She nearly reaches the end of the set without a misstep, when a voice startles her from behind.

Clary whips around to see Jace in doorway, hair windblown and cheeks flushed with the cold of the New York late autumn. He looks like a character swept from the pages of one of her favorite anime—one of the dangerous, adventurous characters.

"You're doing it wrong."

"Excuse me?" She spares only a quick subconscious glance down at her recently acquired workout attire—dark spandex shorts and sports bra—before meeting his intent stare.

"You're doing the moves wrong."

"No, I'm not."

Jace smirks, but it doesn't have any bite. "Here, let me show you."

Clary watches curiously as he shrugs out of his lined leather coat and stalks towards her. She's thrown by his sudden willingness after days of avoidance. Jace has a history of running hot and cold, and—not for the first time—she's having difficulty keeping up.

"Turn around," he instructs her, and she obeys apprehensively. When the strong planes of his chest brush her shoulders, she nearly jumps in surprise. She clears her throat and tries to force down her sudden disquiet at his proximity.

"Your hands are cold," she remarks, shivering as his fingers gently curl into the bare skin of her hips.

"Sorry." The metal button of his jeans presses into the small of her back when he pulls her against him. "Just came in from outside."

In the back of her mind, Clary wonders why the training room was his first stop, but mostly she's focusing on her breathing and trying to keep it under control with Jace holding her the way that he is.

His breath teases the hair at her temple as he speaks. "Strong posture doesn't necessarily mean standing straight. You've got to be flexible, willing to change and adapt."

"But when Robert does it he always looks so still. Steady, like no one could move him."

"Control takes practice, but the motion itself…"

Hands force her hips to dip, and Jace's body briefly moves in tandem with hers as he guides her to the left and then to right in single, fluid motion. Clary knows she's supposed to be doing something with her arms but they hang heavily—uselessly—at her sides. She feels lightheaded with anticipation of him touching her and moving with her like that again.

"Just keep your chest lifted." His left hand abandons her hip to slide up her stomach and splay across her ribs. Clary sucks in a sharp breath that she can't let go.

"Like that," he murmurs into her ear.

And with that quiet encouragement, it's only them. Bits and pieces of reality fall away, until it's just Clary with Jace wrapped around her as though he wants to consume her. She knows he can feel her heart pounding away between them, can see her blush as the heat inside her belly turns beneath the palm of his scarred hand.

Jace's pinky finger dips inside her navel as his lips ghost her neck, and Clary finds herself subconsciously tipping her head to bare more skin to him. The hold on her hip tightens. She becomes very conscious of his body pressed against the length of hers.

"Jace," she whispers, hand encircling his wrist and sliding up the length of his forearm. Muscle tenses beneath her touch. Jace's nose nudges the strap of her sports bra, and Clary swears his teeth nip at her skin. Clary can't stand the teasing a moment longer. She turns her face up to him, but the moment their lips graze, Jace jerks away as if shocked. A stark coldness seeps into her body as he takes several steps backward, hands fisted at his side and mouth pressed into a grim line.

"What—"

"You know what."

His voice is as cold as she feels, and though it makes Clary want to flinch back, she digs in her heels instead. "Stop pushing me away."

He stares back at her defiantly. "It's the only way. I don't want to see you get hurt, and you will get hurt if you get too close to me."

Clary wants to pull at her hair. "How can you honestly think that? After everything we've been through, you still think that you would hurt me?"

"Maybe not on purpose," he says darkly, head bowed. "But sometimes I feel like I can't control it. When I'm around you…the demon in me doesn't care about consequences. He just wants. And he wants you."

"You're not—"

His head snaps up, golden eyes lit like twin matches, and Clary momentarily forgets herself and allows fear to grip her. He closes the distance between them, lowers his face until his lips are nearly grazing her cheek. "You're going to tell me I'm not a bad person for wanting to fuck my sister?" he says slowly, dangerously. "What kind of person sees his sister walk into a room and imagines what she's wearing beneath her clothes and wonders what it would be like to find out? Is that what a good brother does? Fantasizes about touching his sister?"

"Stop," she whispers, looking away from the intensity of his stare.

He nods knowingly, something like triumph shinning in his eyes. "Do you know I dream about you?" he continues more softly. "About us as lovers? Sometimes we're in my bed, sometimes the shower, sometimes right here on the floor of the training room with everyone watching us—"

"You're only saying that to hurt me," Clary cuts in. She feels flushed and disoriented, her mind responding one way to his words and her body another. "You think acting this way will drive me away. Like those terrible things you said to me in Idris. But I know you, Jace, and this isn't just about lust or sex. You can't just take the bad without considering the good parts of what you feel, too."

Jace's face softens almost imperceptibly. His palm finds the curve of her cheek. "You are the good parts. All of them."

She shakes her head, careful not to dislodge his hand. "You're wrong. Someone can't love the way you do and be a monster."

"To love is to destroy," he says hollowly.

"You don't believe that."

"I'm starting to."

[ - ] [ - ] [ - ]

The familiar creak of the elevator descending to the first floor pulls Clary from the dense text of the demonology book she's been poring over for the last hour and a half. Grateful for the potential distraction, she pushes the heavy text across the desk as she stands and stretches her cramped muscles. If she has to look at the description of a palvage demon again, it will be too soon.

She's out of the library and halfway down the hall when elevator's clattering starts up again. The noise stops just before door slides open, and she has to squint in the dim hallway light to make out the huddled group of figures that emerge.

"Keep applying the pressure."

"Yes, Alec. That's the spot. Harder."

"Now is not the time to be cute."

"I can't help the face I was born with."

Clary picks up her pace upon seeing Jace propped up between the stooped shoulders of Alec and Robert as they slowly make their way down the corridor that leads to the infirmary. She catches up with them and almost gags on the pungent stench of blood and sweat mixed with ichor.

"What happened?" Clary stares in alarm at the red-stained cloth Alec is pressing to a wound on Jace's side.

"Jace," Alec says darkly, "acted like an idiot."

"So they maimed me," Jace adds unhelpfully. He looks strangely out of focus, she notices. He's blinking his eyes repeatedly while working his jaw, flexing his hands which hang from around Robert and Alec's necks. His feet alternatively drag or stumble uselessly beneath him.

"Was he poisoned?"

"He? I'm not dead yet."

"By a Curan demon," Alec says over the top of Jace.

An illustrated image from the pages of the book she had just abandoned in the library flashed across her mind. She recalls the description that accompanied it. "He's becoming paralyzed, isn't he?"

"He," Jace cuts in, "has not lost the power of speech. Yet. My toes are just a little more tingly than usual. It's not so bad really."

Robert pushes open the door to the infirmary. "It takes a while for the poison to take complete effect."

"How do we stop it?" Clary asks, anxiously watching the three enter the room and move to the closest bed. Jace sags onto the mattress gratefully. She's not used to seeing him this helpless, and the sight literally makes her stomach ache.

"Maryse is working on an antidote," Robert answers her. "It's fairly simple, but I'm going to make sure she doesn't need anything. Alec, when you're done, take Isabelle to do another sweep of the area."

Alec, who is diligently securing a wrap around the make-shift bandage on Jace's abdomen, nods his understanding without removing his attention from the task at hand. Robert leaves the room after taking a final appraisal of Jace's battered figure.

"Just keep a close eye on him until Mom gets here with the antidote," Alec instructs her a few moments later, the roll of gauze in his hand empty. "He's probably losing feeling in his extremities, and pretty soon he won't be able to move much on his own."

Jace offers Clary a grim smile. "That means you get to stand by with a towel in case I start drooling uncontrollably on myself."

Clary frowns. "Is he still losing a lot of blood?"

"No, the iratze closed most of the wound, but the poison is keeping it from healing entirely. Once he gets the antidote, that'll change." Alec runs a hand through his disheveled hair, looks like he wants to say something more, and then thinks better of it. "Isabelle and I will be back later."

"Don't get yourself killed," Jace calls after him almost as an afterthought.

Alec pauses in the doorway and casts an annoyed look over his shoulder. "Yeah, I would hate to steal your thunder."

Jace watches him leave with an almost pleased expression. "He's getting better at that."

"At what?"

"Storming out after a clever final word. It's not just Magnus's glitter that's rubbing off on him."

Clary shifts—inevitably—closer to him. The nearer she is, the more real he becomes. His hair is matted and not quite as perfect as usual. There's a smear of ichor on his temple, and his skin is covered in a thin sheen of sweat—the only outward indication that he's experiencing discomfort. "What did he mean by that? Alec said you did something stupid."

"That's Alec's version of the events."

"Well, what's your version?" She's close enough to touch him now if she wants (and she does want to).

Jace clears his throat, looks her over as if he's seeing her for the first time that night. "There was a nest of Curan demons down near the docks. We had a plan to surround them and cut them off from the water, where they would have the advantage because they're natural swimmers. As we were getting into place, I came up with a better plan. One that included me stabbing the largest Curan while its back was turned."

"So you were reckless," she says flatly.

"I see you're siding with Alec."

"There are no sides, Jace!" She very nearly stamps her foot in frustration. "There's just you being careless and needlessly putting yourself in danger. I mean…what were you thinking trying to take them on by yourself?"

Needing to do something with her hands, she pulls her stele from the pocket of her jeans and takes a final step toward Jace.

He eyes her warily, although she's not certain what he's afraid of. "What are you doing?"

"My runes are supposed to be stronger than regular ones, so maybe my iratze can do more to heal that gash." She notices that he's stopped flexing his hands, so the poison's effect must be becoming more potent. She recalls what Jace told her on the bank of the river before they attacked Valentine's ship: the closer a rune is to the heart, the more effective it is. So she reaches for the zipper of his gear, pulling it down to the top of his ribs. She pushes aside the thin black material to reveal his lightly scarred chest. Her fingers absently trace one of the thin white lines, and Jace's breath catches.

Clary shakes her head to clear it and replaces her finger with the tip of her stele—right beneath the black coil of a permanent rune on his shoulder. Just as she's about to begin drawing the now-familiar contours of the iratze, another symbol settles in her mind's eye: two curving lines running parallel to each other and bound together in the middle by an oblong circle. Just the sight of it soothes her, and she hesitates only a moment before copying it onto the canvas of Jace's skin.

"That's not—"

"I know."

Jace peers down at the mark thoughtfully. "What is it?"

Clary considers the soft, lulling nature of the rune. The curves remind her of a current in the sea. "I'm not exactly sure. But it should help."

Jace doesn't question her further. Instead he looks her intently, as if the answer is written on her face. "It's how we are. Demons, I mean. We don't worry about getting hurt. We know what we want, and we go for it. Consequences be damned."

Clary sighs. "Is that why you nearly got yourself killed? So that you could live up to your own morbid expectations of how terrible you are?"

Jace shakes his head. "You were there when the Angel told us what I am."

"What Valentine did to you wasn't your fault, Jace." She takes his limp hands in hers, determined once and for all to dismiss any notion that he's somehow inherently evil. "And maybe it made you stronger and faster than other Shadowhunters, but do you really think it changes who you are? You're a good person, Jace. A demon wouldn't have done what you did to stop Valentine."

But Jace is stubborn by nature, and he's gutted by this perceived truth to the point of self-destruction. "You've seen what demon blood does to a person. Sebastian killedMax without even hesitating." His fallen brother's name catches in his throat. "He would have killed every Shadowhunter in Idris if Valentine hadn't kept him in check."

Clary feels cold all over at the implication. "You are nothing like Sebastian. He was a monster. He—"

"He wanted you, too."

Clary stares back at him. She tries to see beyond the candid pain in his eyes to what he's hiding underneath, searches for the ulterior motive—any sign that he's only saying this to push her away again. He can't believe that it's true.

But she sees nothing.

Slowly, Clary releases his hands and steps back. "You think you wouldn't love me if you didn't have demon blood?"

Jace is silent, face washed with self-loathing. Because that's exactly what he thinks.

She bites the inside of her cheek to keep from crying. "That's the only reason you want me." Because she's an angel, and he's a demon, and to love is to destroy.

"Clary…" Jace reaches for her, but she evades him.

"Then what about me, Jace?" she whispers. "What's my excuse for loving you?"

His lips part but seem too stunned to form any words. It's just as well, because the door opens and Maryse enters with a small vile in hand. "Jace, how are you feeling?"

"Fine." And he slides onto his feet as if it to prove it. His eyes, however, stay fixed on Clary.

Maryse's forehead wrinkles in confusion. "But Robert said you couldn't even walk by the time you got back. How could you—"

The rest of the words die behind the closed door as Clary steps into the hallway.

AN: I'm so excited to finally start posting this story! It's been a long time coming, and I bet some of you had gotten tired of me yapping about it whenever I got the chance. Haha.

Feedback is appreciated (and not-so-secretly craved). I can't promise a regular schedule of updates, but I do have a complete outline for the story so it will get finished...at some point. Right now it looks like it's going to be 8-10 chapters long, but if you guys are liking it and I'm liking it, I might be expanding parts here and there. Basically, I'm just looking forward to embracing the whole WIP process again.

I'm looking for someone to regularly beta the rest of the story. I've had a lot of great help with the first couple of chapters, but I don't have anyone signed on to edit/beta long-term. If you're interested, just let me know in a review or shoot me a PM. :)