The girl stumbled over, still hung over, to her mirror. Her legs felt unfamiliar, but yet natural at the same time.

In the mirror was someone unfamiliar to her, with long, jet black hair, a curvaceous and full body, and distinct, but not too large, breasts. It was far from how she looked the previous night, but...

"Y'know what, this is fine. I'm okay with this."

Suddenly, a chill went up her spine, causing her to jolt upright from her slightly unbalanced position. Whatever hangover was left in her brain was immediately dispelled, and she was more awake than she'd ever been. Looking back at the mirror, she saw the source of the sudden chill: a ghastly figure had slid up her back, and was hunched over her shoulder, with similar black hair to what she now had, though she was more...transparent and green than she'd expected.

"Do you like it? It's yours now...sorry, I meant to say it's OURS now."
"...who the hell are you?"
"You don't remember last night? You should've thought twice before attempting that demon summoning ritual on a drunken whim! Now your soul is bound to mine forever, and so is your body! And...just to really rub it in, I made it look more like me, just so it would remind you of your eternal curse every day you look in the mirror! Ahahahahaha..."
The girl seemed unfazed by the demoness on her shoulder and shrugged.
"I mean, I'm okay with that, y'know."
"...hahahahahahaa....wait...what?" The demon's laughter came to a screeching halt, and she seemed genuinely confused. "Wait, no. No, no, this isn't right. You're supposed to be cursing me, damning me to oblivion or something. YOU AREN'T SUPPOSED TO BE OKAY WITH THIS!"
"But I mean....I like this. I like looking like this. I look cute as hell." She did a twirl for the mirror, though the demon didn't move from her position.
"CUTE. CUTE? WE'RE...I MEAN, I'M NOT CUTE, I'M AN INFERNAL ENTITY. I'M SUPPOSED TO BE STRIKING FEAR INTO PEOPLE. Besides...you were male last night! This shouldn't be pleasurable! I've seen my sisters curse people like you before, and they ended up tormented by the situation you're now in!"
The girl scowled. "...Hey. Demon lady. You're bound to my soul, right? Surely you can see what's going on in my brain too, right?"

There was a pause. The demon put her hands to her face.

"Dammit. Of all the people to take as a host...I'm terrible at this 'doing evil things' thing. My sisters always made fun of me, but I was just happy to finally get a chance, and..."
"Hey, it's okay. Look, I think this can be a nice mutual thing we've got going here. I get to keep this, you get to do...whatever it is demons like to do when they're possessing people, everybody wins here."
The demon looked away in embarrassment. "Fine. Okay. I get to have your body to cause general chaos twice a week. Is that a deal?"
The girl nodded. "Cool with me. No killing though, okay?"
"....fine. No killing. Hurting okay?"
"Only if they deserve it."
"Whatever. Works for me."

The girl reached up and petted the demon on the head. The demon responded by burying her head in the girl's shoulder.

"....now I just hope my girlfriend will be okay with this."
"I can make her okay with it in exchange for taking your body on holidays."
"No deal. I'll let her decide on her own."

The girl hopped away towards her closet, the demon trailing behind her like a ghostly tail. The rest of her life was gonna be pretty nice.