A fire naturally occurs when the elements [heat, fuel and an oxidising agent] are present and combined in the right mixture, meaning that fire is an event rather than a thing.
fuel.
kindling.
You place the first bit of fuel down the first time you see her, it starts when her flame is burning the brightest, threatening to set it alight too early to have it burn out too quickly. You place the first bit of fuel down when she attacks Amber – poor, innocent Amber – and takes half her powers. You carefully pat the fuel into place and shield it from her flames when you ultimately lose to her. Because you never really stood a chance.
You were never fond of the colour red, not since Summer died and Raven left, but Ruby had made you start to love the deep, dark shades of red. But this woman showed you a whole new appreciation for the colour; she showed you the burning, intense shades that you never knew could exist in a colour that has been tainted for you by far too many losses in your life.
You used to see the world in shades of black and white; right and wrong. Now it's all the glowing red of burning embers that fly flickering from a raging inferno, with golden sparks the colour of her eyes and you want to plunge yourself into the fire and fan the flames.
body.
You lay down the largest log, the largest bit of fuel that the fire needs to truly grow and continue to burn when you are told to go undercover to gather Intel. Gaining a spot in her inner circle isn't very hard; you are a skilled hunter after all – she has firsthand experience, much like Emerald and Mercury do – and with your sisters rogue reputation, all you have to do is say you've agreed with your sister and turned your back on Ozpin – which is now true, who would've expected two rogue Branwens? –, and they are all more than happy to accept a rogue Branwen.
She smiles at you the first time you meet her and you wonder if she really does remember you – you tried to kill her after all – because God you don't think you could've forgot her even if you tried. That deep red of her dress – like your cape becomes in the moonlight – and even with her hair grown out – a good look for her, and you wonder what it would be like to run your fingers through it and you've got to stop wondering things like this – she still looks just as beautiful as the first day you saw her.
Except now she crackles as she walks, and you can feel heat radiating off her when you pass each other by, or when she calls you over to get intel from you about Ozpin – you give him "intel" of course, just enough for them to trust you to not be compromised – which you are – but with just enough mistakes that they'll never catch her – and you wonder how it would feel to touch her skin, but you think you might get burnt.
heat.
ignition.
You light the first match and throw it onto the fire when you kiss her for the first time one night after a mission goes particularly well – you should feel bad about the things you've done, they go against your lessons as a Hunter, but you don't. You say you kiss her because you initiated it; the two of you had been dancing around the topic for month's now, waiting for the first spark to fly and now it has and what you've spent months gathering the fuel for has ignited.
You may have thrown the first match onto the fire but she is the one who fans the flames, she grips her fingers into the back of hair and pulls you with her into the privacy of her room. She pulls you closer and your lips crush together and you're gasping for air like a dying man in a burning house with only smoke to breathe while she is collected – like always – because she is a burning, blazing fire and she takes the oxygen you breathe and strengthens herself with it.
She smiles at you – so deceptively sweet – between kisses and says that she's been waiting so long to do that and all you can do is moan into her collarbone and shake all thoughts of Ozpin and Raven and Taiyang and Ruby – poor, innocent Ruby; deserved better than a dead mother and a father who shuts down and an uncle who betrayed her and a legend to live up to that she doesn't even know about yet – out of your head.
This is what really starts it all.
oxidising agent.
It blazes out of control the night Penny dies and as Vale burns around the two of you.
She's happy, so incredibly happy, you've never seen her so happy in all the time you've known her. You've seen fire burn behind her eyes before – all entrancing hatred and fire and anger and pure, unadulterated rage – but never like this. This is the fire of the sun, screaming to everyone I am here, witness my power.
The sparks jumping from Penny's body, you decide, are the sparks that light the blazing inferno inside her.
She pushes you against a wall – you can hear the screams of the civilians and the Hunter part of you that would've leapt to action burned to ashes long ago – and kisses you long and hard, her teeth pulling at your teeth and you melt into the heat she's giving off and push back into her, running one hand through her black hair falling around her like ashes, and the other down to the small of her back. She pulls away from you, slightly flushed as the ground shakes and the hill cracks open, and you see her retreating figure head towards the tower, and you look up to see Emerald and Mercury filming and what looks to be Neo falling through the sky – odd, she'd never leave Roman, not now – and there is fire burning all around the city and it looks beautiful.
You can hear heels hitting the pavement behind you and you turn around – expecting Cinder again although you know she won't be back for a while – and Glynda is standing there, frowning in her trademark I'm not angry, I'm just disappointed way and it almost makes you feel bad. Almost.
"Qrow" is all she says at first, as the silence – or as close as it can get to silence as the city burns around you – hangs in the air "What are you doing? Being involved with her, really?"
Your words catch in your throat like smoke, deny it your brain screams so you do "I don't know what you're talking about" you say as you stare at her.
"Your collar is singed" she says and you freeze.
You don't know what to say, you don't know what to do. You would rather not fight Glynda, she was never outright rude to you, but you can tell by her stance that she's getting ready to attack you. So you do the only thing you can think of.
You fly away.
embers.
You sit perched atop a jagged end of the tower where the dragon sheared off the roof, and you watch her. You can tell she got the last half of the power from the way her fire flickers – not to say her fire abilities weren't always gorgeous but this was on a whole new level – and the air crackles around her and she reaches down and cradles the young red heads face – Pyrrha, you think that's what her name is, you'd heard of her of course – before she crafts her bow and arrow out of thin air as it melds together.
Then she fires.
The few seconds before Pyrrha starts gasping is the closest thing to silence you'd heard in a very long time.
Then Ruby comes bounding up the side of the building - poor, innocent Ruby; deserved better than a dead mother and a father who shuts down and an uncle who betrayed her, a legend to live up to that she doesn't even know about yet and who witnessed two of her peers killed in front of her very eyes – and she screams.
The dragon freezes despite the heat in the air Cinder is radiating, but you grab Cinder – you practiced this for months, being able to master the ability to pull someone else with you – and you fly away, leaving your niece alone on the roof and you don't look back.
Cinder is hurt, that was inevitable. You may have gotten her out as quick as you could but you could only fly as quick as a regular crow can and even that wasn't quick enough to defend her from the power Ruby had unleashed.
She lays on your bed in the small, isolated cabin you own, her chest rising and falling in a slow rhythm, enough to make you feel confident she'll be ok.
Her eyes flitter open later that night; a week after the Battle of Beacon and you let out a relieved breath you didn't know you were holding when you make eye contact with her and can still see the fire burning behind her eyes – it's a smaller fire now, a low burn with flames crackling and embers spitting out – and as she grabs both sides of your face and pulls you in for a kiss – a slower, sweeter one. Gone is the harsh urgency from before when she was constantly busy and planning and scheming, now she can afford to be slow and take time – a warm, comforting heat spreads through your body.