This is basically a mishmash of headcanons for what may happen when Caitlin becomes Killer Frost on the show.
I should probably put trigger warnings here for attempted suicide and mild violence/gore.
It took her a long time to come to terms with it. There were times when he caught her staring at nothing or, even worse, her reflection, where he knew she only saw a stranger. It was in those times when Barry was more than tempted to pick up the phone and beg Oliver to talk to her, to try and make it better, but he knew, really, what they would both say.
Give her time, Barry. Oliver would sigh.
I'm entitled to be angry, Barry! Caitlin would snap.
They'd both glare: You've never killed anyone, Barry!
But Caitlin... oh god had she, and didn't she just hate herself for it? When her hair turned white because an open door let the draft in. When her lips turned blue when she drank Cisco's iced water by mistake. When the media started calling her Killer Frost.
When her tears fell, but they froze the ground they landed on, and that just made her cry harder.
Barry was always warm when he ran. His cells moved quicker and friction warmed him, as well as elevated heart rate and calorie burning. So they'd agreed, in early days when they were a team but not yet friends, to set the thermostat below room temperature, somewhere comfortable for everyone. It wasn't an issue.
And then the sub-zero temperatures nearly killed Caitlin and it became an issue.
She wouldn't let them turn the heating up, even going as far as to freeze the thermostat so that they couldn't change it, refusing to be a burden to anyone but herself. So Barry had come in the next day, sick and tired with worry from watching her skin turn from pink, to white, to blue, to translucent, wrapped in a thick coat, scarf and gloves in the early November heat. He shrugged them off, hot and clammy, and wrapped her up in them instantly, plopping the fluffy hat on her head with a smile. She grinned at him, whispering a silent thank you, and he wanted to hug her, wrap her up in his arms rather than his clothes and promise her everything would be alright.
But the last person who touched her fell dead from her embrace, and he knew, Caitlin Snow would never take that chance. She was a Doctor. Not a killer.
The snow and ice that winter brought was deadly, not just dangerous. She shook, all the way through Barry's clothing, and Cisco's, and Doctor Wells' and sometimes Joe's, when they tried to help. They brought her coffee and hot water bottles, tried to keep her up and moving, but always to no use. Her cold spells went hand in hand with migraines when they couldn't dispel them quickly enough, and she tucked herself away in the cupboard near Barry's treadmill, alone and cold, willing the pain to leave. No one could help her then, no matter how much they wanted to.
Ronnie had trouble accepting it when they found each other again, the glowing skin and white hair freaking him out slightly. But when her cold threatened to extinguish his heat, he kept his distance, admitting that everyone would be safer with her filling the hero shoes, and that he'd always be there if they needed to thaw the city. She stopped wearing her engagement ring a short time after that.
Joe fetched them all coffee one time, complaining repeatedly that the new guy at Jitters made it just too hot. Barry took the chance when he saw it, and stole the polystyrene cup from his hand, and dumped it into hers. She blinked, heat hitting her more than the caffeine of her own drink, and it was gone again, back in Joe's hand across the room, and cool enough to drink.
Those were the good days.
Nobody talked about the bad days, when people had died, again, because of her, and she'd stayed behind after everyone had left. Barry found her the next morning, her backless suit shredded, her hair white and her hands and eyes glowing, curled in a ball at the bottom of a freezing cold shower. No one had a clue how she had survived it, and she screamed when she found that she had.
They stayed cautious around her for a while, staying as close to her as possible, helping piece together the fractures in her heart bit by bit. And Barry… he wrapped her up, ignoring her worried protests, pulling her head down against his chest, over his fast beating heart, guava fruit shampoo hitting his nose as her sobs hit his soul. She stopped crying eventually, her arms coming up around his back, clinging to him tightly. She took no heat from him, and he grinned down at her. Control. Progress.
She started coming back to them after that.
(She spent valentines day wounded up in a ball in Barry's bed. The open back of her suit left her vulnerable to Captain Cold's gun, and he was too worried to even realise that the only time she woke on February Fourteenth, he brought her tea and curled himself around her, sharing body heat, kissing her forehead as she dozed back to sleep, whispering promises that Leonard Snart would never hurt her again. He fell asleep after her, her leaching the heat he spilt out in excess, and he gripped her tighter, dreaming of carrying her limp body from where she fell to where she would be safe. They don't talk about that either.)
Spring dragged itself to Central City slower than normal, but finally, it arrived, bringing a happier Caitlin in it's wake. And her layers receded, until warm pink flesh peeked out at the sun again, sucking in all the heat she could hold in her tiny vampire-y body.
The sun brought out her smile again, in times least expected, like when Cisco had teased her once too often and all it had taken was one little poke, before he was suddenly sporting a lapful of melted ice cream, which shouldn't have been possible. (Barry cornered her about it later, where she admitted to pulling head from the air through the ice cream. It was a neat trick, but she asked him to keep it secret, because she didn't fully trust Doctor Wells at that moment. And so it remained, their secret.)
Guillermo Barrera's attack on Central City was more than worthy of his name. People with something to hide turned up all over town, the secrets tortured from them by a man dressed as a gargoyle. When the assassinations started (with no ties to the League of Assassins, he'd had Oliver check) they redoubled their efforts to find him. Only Brutale found them first. He intercepted the return trip from playing firefighters, Caitlin warmed from the flames and Barry laughing with delight at a job well done. Adrenaline made everything blur, but once his blade was at her neck, the Flash knew red. Barry charged, slamming him against the wall at high speed to knock him unconscious, seconds too late as she fell, her throat cut in a pile on the floor.
He wailed, the police, public and media getting a good look at the distraught hero, as he collapsed at her side. But she turned to look at him, pain alight in her eyes as she reached for him, and giggled her way through a flippant comment of how she'd have to expand her scarf collection. The blood froze in the cut, sealing the wound closed and keeping her alive. He'd sobbed, touching it gently, and hugged her tightly.
Losing her to this was not an option, and finally, Oliver's philosophy made sense.
Starling City cried for help, and the Arrow reached out in return when the price on the heads of the people he held dear grew too large. She bound down the stairs behind him, her hair glowing purely to watch the jaws drop. The reports of the Frost Bringer and the Streak appearing with the two (three) marked vigilantes to hack their targets to the ground stirred up commotion nationwide. They set up shop temporarily in the foundry, stealing the emergency blanket and the Salmon Ladder alike until Iron Heights gained itself a new inmate. Felicity offered Caitlin her own manikin before they left, promising to have it ready for her before their next visit. She hugged them all gently, breathing shallowly, expecting each one to drop to the floor and shatter like ice. And Roy got a small peck on his blush cheek for his supportiveness, so Barry made sure to knock him to his arse as he ran, scooping her up and listening to she screech of delight as he ran.
She became more hyperactive when summer wrapped its clutched around them. She froze ice cubes instantly when Cisco whined at the heat, froze drinks to subzero temperatures before pouring them over his head to make him scream. Cooled the rooms and sat with her delicate hand on the computers when they overheated. When the year reached it's warmest, she bounced on her feet and hugged him, overjoyed at eating ice cream for the first time in nine months without going into shock. He held her tight, laughing into her hair, kissing her temple. Her giggles swelled his heart, and she kissed his throat - the only patch on skin she could reach - before she let him go. Cisco grinned at him as she left.
Cisco, as loveable as he was, got annoying at times. His insistent need to name the Meta Humans began to grate on Caitlin's nerves when she'd had a particularly bad day, and even more so when he started trying to rename her. There were the lame ones, the ridiculous ones, the ones that they ignored to spare his pride. But when Barry returned to S.T.A.R Labs to find Caitlin glowing with anger, he knew the puns about her name had begun. When they left twenty minutes later, suited up for the expected fight, Cisco's trousers were still frozen to his seat for that particular Disney reference.
They decided to stick with Killer Frost.
There was a new girl at Jitters - Melanie, according to her name tag - , who batted her eyelids at Barry, whenever she saw him, flirting outrageously. He never bothered with flirting back, since idol chit chat had revealed that she didn't care much for the Flash (Not that that was a selling point, but still). He was polite, of course, but it was hard to keep his focus on the dip-dyed teenager with Caitlin by her side, insisting that she could buy her own coffee. He paid for both anyway, because he didn't like the angry glint in her eye went the girl spoke down to her. It was still there when she performed her silly little flirting routine.
But the next day, when Iris' friend took their order because Melanie had been late for shift, claiming the engine of her car, had frozen, in the middle of August, no less, Caitlin grinned, slipping her fingers through his as the blonde rushed through the door.
Her birthday was warm. Loving hugs and woollen jumpers given everywhere you could look. A surplus of food and coffee set out across the surfaces of Barry's apartment - because even before, Caitlin had been addicted to coffee. She teased him, waiving the alcohol she drank in his face, because while she didn't have his super-healing, she also didn't have the same metabolism as him, so alcohol worked. And she took to wearing the sapphire necklace she was gifted with (her most expensive present. Sometimes knowing billionaires - or middle class citizens sleeping with billionaires - was a soul crushing experience) whenever she could get away with it, the blue tear drop laughing at them, symbolism screaming at them. Iris pointed it out, that it looked like ice, and he lead her from the coffee shop with a hand on her spine and a soft spoken Miss Frost in her ear. She shivered, but for once, she was warm.
She trekked down to the cemetery, frozen ground cracking under her weight. Things were better. The media published an article, the one that he printed and framed for her birthday present, citing her as a hero, and the name Killer Frost was rarely used any more. She showed up with the Flash at crime scenes, kicked arse, and little girls and boys looked up to her. She led flowers down on the graves she stopped by, a bunch of Verbesina virginica - more commonly known as Frostweed - perched against the headstones, each glistened, frozen, preserved.
They were the six who had died at her hand in her first few days, three one night, two the next and the last on the third. They were those who warmed her, allowing herself to craft her to who she wanted to be, who she was now, and she thanked them through her grateful tears.
He led her away with an arm wrapped around her shoulders, her head lolled against his shoulder with a somewhat lazy smile of her face. Her fingers pressed against his collar when he made her laugh, leaning on him to keep herself upright. He knocked her chin up with a single finger and stooped down to kiss her lightly. She sighed into him, like she always did, and he arms went around his neck, pulling him in close and breathing him in deep, as he did to her, fisting his hands in her currently brown hair.
Yes, Killer Frost had ice in her veins.
But Caitlin Snow lit a fire in the Flash's that he never wanted to extinguish.