Disclaimer: I do not own Adventure Time or make any profits from it.
Marceline closed her eyes and smiled proudly. "You look good, flamey."
Flame Princess walked with an uncomfortable expression on her face, moving her huge hips in the exaggerated sway Marceline had shown her, clearly expecting to fall over every other step. First spending most of her life isolated in her kingdom, and then further isolated in a prison of glass, she had no idea how to move in the ways to get attention or command with her presence alone. Finn, and to a slightly lesser extent, Bubblegum, had been her mentors.
With them as examples… Marceline stuck her tongue out at the thought of it. Good enough for running full tilt like a golden bull at anything that needed smashing, or scuttling around lightning-quick for half a dozen things that needing doing right away, but it wasn't really a way to look cool.
And now Flame Princess stopped in front of Marceline, wearing clothes she had chosen with the Vampire Queen to help; not picking for her, but choosing with an eye towards what was not fashionable and would look good on her for long times to come (or at least until another growth spurt doubled her in size). The two of them together in a small thrift store run by a mad scientist who was into fabrics in a big way, the coat-racks twisted behind them at odd angles and into absurd dimensions, casting the shadows around them in bizarre shapes. Flame Princess was her own light, and she glowed brightly as she got a good look at herself, smiling a little bit at the sight of herself. The refined clothes she was accustomed to had been cast aside in favor of the more utilitarian and extremely retro attire Marceline favored, and the fire elemental girl plainly liked it (though perhaps it was just because Marceline had picked them out, and Flame Princess craved approval so dearly she would instantly agree to nearly anything just for that brief moment of connection and togetherness).
Leather pants, made from dragon-hide and dyed a deep black, wrapped snugly around her thick thighs, the fabric smoothing out as artful tatters around her knees. She seemed a bit uncomfortable with how well they outlined the exquisitely large roundness of her backside, but there was so much to define that the sheer volume alone was charming. Flame Princess tilted herself around, examining the angles of her thick, strong legs, and carefully eyes the short biker boots encasing her proportionately small feet. She adjusted herself from foot to foot, looking herself over.
The edges of a jacket-coat, long and made of the same dragon leather as her pants, flapped against the tops of her boots. Voluminous, adapted to her excessively curvy body and threaded with the images of things Flame Princess held dear to herself (not the signs and designs of the Fire Kingdom, that was too expected, but of smaller and personal things; the locks and broken bones of dungeons she explored with Finn, the shimmering gauges and metal of Bubblegum's laboratory, the music notes of Marceline's music, the stretchy outlines of Jake's amazing powers, and the whispering edges of the flames that embodied the passion she felt for these things). It was colored a dark and peaceful shape of blue, and like the rest of her attire, smoked and shifted in peculiar ways like living flame; ensorcelled to be immune to fire, it had taken on some properties of fire itself. Flame Princess walked around, trying to get used to its weight on her, and smiling fondly.
A t-shirt, as red as dull flames and emblazoned with a logo for a band called the 'Scream Queens' (and Marceline had given this to her before in secret, after autographing it and making her promise to lord it over all the envious fanboys and fangirls who didn't have an autographed shirt from Marceline for themselves), was snug on her body. It rippled with its own heat, the heat of deeper things than mere fire, but it was perhaps too snug; ending slightly above her belly button, Flame Princess looked down doubtfully in front of a handy mirror growing right up out of the ground. Framed between the edges of her coat and the midriff of her shirt was her pudgy belly, hanging over her beltline and swallowing up the button and top of the zipper.
Flame Princess tilted her head, puffing out a cheek. Full lips turned out, and she looked uncertain as she absently poked her belly-pudge. A mind spent in loneliness craved approval. "Marceline?" She asked, timid. "Do I, y'know…"
"Yeah?" Marceline said, with none of her usual bravado. It was quiet, waiting. She looked down at Flame Princess, shy for all of her famous tantrums and mood swings, at where the weight of her breasts considerably deformed the imaging on the shirt's front and hoped that her cute friend didn't have any body image issues.
Then again, Marceline considered, Flame Princess hung out with Finn and Bubblegum all the time, who were respectively chubby in a 'big beautiful man' fashion and chubby in a 'tall and curvy and busty' way. People typically defined what was normal by the people around them, or so Marceline had heard, their sense of social appropriateness and medians conceived by the Brownian motions of being bounced around by their social circles. Given that, she thought that Flame Princess almost certainly didn't think much about her size, if she thought about it at all… though she also thought that, since she thought Finn and Bubblegum were normal, the girl had a lot more problems than she thought.
Flame Princess, oblivious to Marceline's musings, looked herself over in the mirror. She thought about a time the other day when Bubblegum had taken her swimsuit shopping (despite the unlikelihood of her finding a medium to swim in), and had picked out what Bubblegum had approved of; a modest black two-pieces swimsuit, with flame designs upon the cups, yellow at the bottoms and shifting to red at the stylized edges.
She'd looked really good, Flame Princess had thought, and she smiled with just the hint of a smirk, basking in the approval of her friends as though it was the heat of the sun, and fire elementals craved heat-
They needed the heat, to live.
And it was nice, Flame Princess thought, to be taken out to find clothes for her personal style, to get away from the attire of her homeland and expand her look, just as she was expanding her personality. (And, for that matter, expanding in other ways; her heritage, going all the way to the mighty fire giants, was showing, and where once she had to look up into Finn's face now his eyes were directly on level with her expansive chest, and she glowed brighter just thinking of the way his face lit up when he looked up at her and smiled like she was the most perfect and sweet woman to have ever blazed.)
Flame Princess burned a little brighter than she ever did, and Marceline looked up at the brighter heat. Flame Princess grinned, a little shyly, and she said, "I really love this look."
Marceline grinned and clapped her on the shoulder, her own coat swishing in sympathy with Flame Princess. "Knew ya would."