Set after 1x06 Red Faced.

Because I refuse to believe Alex told Kara exactly what happened in Morrow's secret warehouse lair.

I listened to Heartbeats by José González on repeat whilst writing this, hence the title.

"Ms. Danvers," Hank greets you stoically, looking up from the panel in front of him, giving you a slight nod as you walk into the DEO control room. You roll your eyes good-naturedly, gesturing to the symbol on your chest, but Hank has already directed his gaze back to the touchscreen under his fingertips.

"Director, please," you say, with mock exasperation in your voice, striking your best imitation of a superhero pose. "When I'm in the threads, it's Supergirl."

God it feels good to have your powers back.

Hank turns to face you properly, crossing his arms over his chest and settling into a casual stance of appraisal. He looks entirely unamused, but you know better. The corner of his mouth twitches slightly as he bites his cheek to keep a smile from breaking his stony façade. After a beat, he inclines his head towards the training room. "She's in there with Vasquez," he states, and you give him your brightest smile, unabashedly skipping towards the security panel that will grant you access to said room.

You're absolutely positive he knows your super hearing can pick up the sharp exhale of air that constitutes a Hank Henshaw chuckle.

The lead-lined walls that encompass the training room slide open with an emphatic whoosh, and a cacophony of sounds spill out.

Heavy breathing. Grunting. Hearts pumping. The shuffles of perfectly-timed footwork across the floor.

Your brow furrows as you step into the observation chamber and watch two figures sparring absolutely ferociously. Kicks are flying fast and furious, fist and forearms connecting with a rhythm to which only the two of them seem to be able to move. It's beautiful and completely terrifying.

Alex throws a quick jab at Vasquez' nose, but it's stopped by Vasquez catching it in her right hand and throwing an equally fast jab right back at Alex's jaw.

The resounding thud of bone on bone stops their brutal dance and makes you jump. You swallow a yelp, not wanting to distract either of them. Alex stumbles back looking dazed, but Vasquez doesn't take advantage, instead looking-wide eyed at the woman in front of her. "Oh fuck, I'm so sorry Agent Danvers," she says, breathlessly. "I thought you'd- I mean normally I have to be so fast- I just-"

She's cut off by Alex bearing down on her, a powerful right hook heading directly for her ribs. Vasquez jumps back, barely evading the blow. Alex throws a left uppercut, and Vasquez slips to the side, slightly off-balance.

Alex growls with frustration. "Counter, Vasquez!" she snaps, sending a right cross straight for the woman's throat.

You see what's going to happen before it does.

Alex's fist connects with your open palm, and her eyes go wide. "Kar-," she cuts herself off quickly. "Supergirl?" She finally realises where her knuckles have landed, and her eyes harden. "What the hell are you doing?" she grinds out from between clenched teeth.

"What are you doing, Agent Danvers?" You snap right back. "Are you trying to put Agent Vasquez in the hospital?"

Alex wrenches her fist from your grip. "Vasquez can take care of herself," she seethes, glaring down at you.

You can hear Vasquez' heart beating a little too fast, and the slightest rustling of fabric tells you she's shaking.

Alex slips back into her usual cool demeanour like she didn't just almost crush a woman's windpipe, looking over your shoulder at the nervous figure standing behind you. "You're dismissed," she says, contempt coating her words.

Quick footsteps, another whoosh, and Vasquez is gone, leaving you and Alex staring each other down again. She crosses her arms over her chest and raises an eyebrow at you. "So," she drawls, "care to explain why you interrupted a routine DEO training session?"

"Routine?" You give her an incredulous look. "What has gotten into you, Alex?" A bruise is blossoming on her jawline, but her face remains emotionless. "You could have seriously hurt Agent Vasquez!"

That gets a substantial reaction from her. She throws her hands in the air and looks up at the dormant kryptonite emitters in the ceiling. "Kara," she says, condescendingly. You scoff, but she ignores you. "Vasquez is a fucking field agent. She has a black belt in jujitsu and is proficient in three other disciplines of hand-to-hand combat." She looks back at you, dangerously close to showing you the same contempt she showed Vasquez mere minutes ago. "Just because I have to use slightly more force in here with her than I do you, doesn't mean I'm going to 'seriously hurt' her."

You frown, a small pang in your chest at the subtle insult to your fighting skills – or lack thereof. Alex knows you've been training like crazy, and she's normally so proud of that. For her to use it against you now…

"She's still a human being, Alex," you say, unsure, feeling – for the first time – that you may actually have done something wrong by protecting an innocent person. "She's…" you want to say fragile, because she is. All humans are. Even Alex.

But suddenly Alex is looking at you like she's daring you to continue that sentence, so you stop, unsure of how to proceed. You tug at your cape, fiddling with it, feeling the material slide between your fingers.

"And I'm not?" Alex challenges. You see a muscle in her neck jump.

"What?" You recoil, slightly annoyed. "That's not- I didn't mean it like that, Alex." Now it's your turn to throw your hands up in exasperation. "You know I didn't mean it like that." You take a step towards her, staring intently, looking for an answer to your very first question, "What is with you today?"

Your answer is a sly grin crossing Alex's face. "Fight me," she simply says.

"Uh..." you step backwards again, feeling a small tremor of fear trickle down your spine, but you're not quite sure why. You've never really had a reason to be afraid of Alex, and you don't think you have a reason now – she's probably just joking – but the way she was trying to get at Vasquez was so unlike the Alex you know. The Alex you know has a dry sense of humour, sure, but today she just seems all over the place.

"Kryptonite emitters at five percent," she says, voice steady, "no more."

You realise she's being serious.

This chases the fear of Alex from your gut and replaces it with fear for her. "Alex…what? No, that's crazy!" You've never trained together with the kryptonite emitters below fifteen percent, and even at that level you left some pretty substantial bruises on her body. "Absolutely not!"

But she's already hitting buttons on the spec panel. Seconds later, the room is awash with a dim green glow and you feel gravity take a slightly firmer hold of you.

Alex stalks towards you, and you respond by backing away from her, resolutely shaking your head. "Alex, I'm serious. I'm not doing this with you."

"Yes you are," she states, a predatory grin your only warning for what comes next. Then she's throwing a flurry of punches at your head. The kryptonite prevents you from flying, but you clear her range of attack by jumping the full diameter of the room in a single leap.

But she's on you again in seconds. "Alex! Stop!" you yell at her, ducking under a roundhouse kick that she was very clearly aiming at your head. A second one targets your ribs, and your throw an arm out to block it. You barely feel the impact, but Alex winces and stumbles forward, steadying herself about an inch away from you.

"Five percent," she repeats, already panting. Her breath causes a few stray hairs to move across your cheek. "I can't hurt you."

Then there's an elbow coming at your nose, and you've had enough. The kryptonite dampens your speed, but you're still fast enough to catch Alex's arm and give her an almighty shove, sending her to the ground.

But she's as quick as ever, and rolls on her shoulder, getting back onto her feet within a second. You finally put your fists up.

It just makes the fire in her eyes burn brighter. "Nice counter," she teases, smirking. "But you'll have to do better than that to keep me down."

"I don't want to keep you down!" You slip right to dodge a jab. "I don't even want to fight you!" Slip left to dodge the subsequent cross. "Alex this insane! You're going to hurt yourself!" Jumping backwards to avoid a kick to the solar plexus, you catch a strange expression flick across her face.

"Planning on it," she mutters, and you're not sure if she knows you can hear her. You drop your guard, dumbfounded, trying to decipher what the hell she means by that.

But then there's a palm heading straight for your nose – instinct kicks in and before you can stop yourself, you've blocked and countered with a little more force than you meant. Alex grunts and stumbles back, a coppery tang filling the air.

You flinch at the trickle of blood coming from her split lip, horrified. But she's grinning, and her tongue swipes over it, savouring it.

"Oh- oh god. Alex," you stammer, pressing your hands to your mouth, shaking your head.

Alex's grin slowly disappears as she focuses on you, replaced by a worried frown. Insecurity clouds her eyes. "I'm fine, Kara," she says, gesturing to the blood that continues to seep from her lip. "It's nothing. Keep going."

You let your hands drop slightly, still shaking your head. "No," you breathe out. "No." More emphatically this time. "Enough. That's enough."

"Kara-"

"I said no, Alex!" You snap at her. She looks taken aback. "Look at what I did to you." You can't take your eyes off the blood now trickling down her chin. "This has to stop." You're angry at yourself for not being able to keep the pleading tone from your voice.

Alex doesn't even try to hide it. Her transition from calm to desperate is jarring. "Kara please," she begs. "I need you to do this."

Now you're even more confused than before. "Why?" you ask her. A wave of anger twists your insides. "Why do you need me to do this? You know I hate-" your voice catches on the lump rapidly forming in your throat, "I hate hurting you. You know that!"

Alex lets her fists fall to her sides, and her shoulders drop. "Exactly," she begins, and the noise you make is a cross between an exasperated grunt and a sob, but she keeps talking. "That's exactly why I need you to do this, Kara. You hate hurting me. You hate hurting anyone." You're sure that your incredulity at her nonsensical non-answer to your question is clear on your face, but Alex keeps talking. "That's why you're the hero and I'm…" she trails off, squeezing her eyes shut briefly before finishing. "I'm not."

The sudden defeat in her tone catches you off guard, and your anger melts away. "Alex-" you try interjecting, but she cuts you off.

"You're the good guy and I'm the bad guy, Kara," she says, shrugging, as if it were the most obvious thing in the world. Your heart clenches as she levels a soulful gaze at you. "The good guy beats up the bad guy," she continues, "That's how it goes."

You've heard enough. Taking three strides towards her, you grab her shoulders. "You are not the bad guy Alex. That's ridiculous and I can't even begin to understand why a thought like that would even cross your mind, let alone stay there long enough for you to insist on forcing me to- to hit you."

You're surprised to find her eyes are somewhat glassy, and you realise she's holding back tears. "Kara, I'm-" she inhales, steadying herself. "I've been keeping something from you. Since the Red Tornado incident."

You make what you hope is a gentle, supportive noise in the back of your throat, but you're well-acquainted with how much it can hurt when Alex reveals the things she's been keeping from you.

Her eyes move to look at the ceiling. "Dr. Morrow…" she chokes out. "Kara, he's not in DEO custody." Her eyelashes are wet when she looks at you again. "He's dead."

Your eyes widen. "Dead?" Confusion keeps clouding your mind and it's starting to frustrate you. "But he was controlling Red Tornado remotely. You went to apprehend him."

Alex just stares, waiting for you to realise…what?

Then it dawns on you, and you repeat the words you heard two days ago in a stunned whisper. "'No commands are being transmitted to the android'…Alex, what did you do?"

Alex finally gives you a straight answer. "I shot him. Right through the chest." She swallows, and you feel her trembling under your grasp. "I watched him die, Kara." Your stomach churns, and you choose to blame the kryptonite for it. "I. Am. The bad guy." She punctuates each word with a sharp prod to the chest with her right index finger.

You feel like you're flying upside down. Inside out. Blood rushes to your head and your ears are ringing. You're sure of nothing and everything at the same time. You're not sure if this is really Alex – your Alex – standing in front of you, chest heaving, searching your face, looking desperate. But you are sure that everything she's ever done has been to protect you, and you are sure that you trust her.

So you grab her hand from her chest and pull her against you, threading your other hand into her hair. She slumps, relieved, clinging to your shoulders, resting her full weight on them. The kryptonite emitters are still on and she's heavy, but you hold her up, feeling hot tears dampen your neck. Alex is a silent crier – she always has been – but it doesn't make your heart tug any less painfully at your chest.

You whisper your next words directly into her ear. "No. You. Aren't." Each one punctuated with a kiss to the slightly damp, salty skin of her jawline.

A watery chuckle rises from her spot in the crook of your neck, and you smile a little, glad she appreciates your cheekiness even in this emotional state.

"Alex, you're the good guy," you say softly, but with conviction. You hear her inhale, ready to rebuke, but you don't let her speak. "I'm sorry you had to kill Dr. Morrow. I'm sorry I didn't know. And I'm sorry you had to keep it from me until now." You sigh heavily, General Lane's words echoing in your head. The whole debacle could have been avoided if you'd just kept your temper in check in the first place. "I'm just…so sorry."

Alex exhales shakily and pulls back to look you in the eyes. "You're not upset?" She squints slightly, "I mean, I killed someone, Kara. You and Superman…" she waves her hands aimlessly. "You're heroes. You have a code against that."

You won't lie – her words sting a little. But only because she's right. Taking a human life is something you wished neither of you would ever have to do. It's definitely not something you ever imagined Alex even capable of doing. Sweet, gentle Alex, who used to carefully carry spiders and bugs from your bedroom without hurting them, even though they terrified you. Alex, who would literally never hurt a fly.

You hate that she had to change that for your sake.

"Alex," you start, suddenly aware of how corny what you're about to say is going to sound.

You say it anyway. "You're my hero. And I trust you. With everything. I trust that you didn't-" the word sticks in your throat, "-kill Dr. Morrow without a good reason." You're not sure if there's ever a good reason for killing someone, but you're a little too overwhelmed to explore the implications of that right now.

A tiny smile lifts the corners of Alex's mouth, and she relaxes slightly. She opens her mouth to say something, but appears to change her mind a second later.

You fill the silence for her. "I love you," you say, emphasised with a kiss to her temple.

Alex looks like she's going to cry again, fighting her emotions to return the sentiment without her voice breaking over the words. You take pity on her. "I know. You love me too," you quip, winking.

She laughs, the sound full and rich in the hollow room, and even though the kryptonite emitters are still on, your heart soars.