Aini was quite literally sick of summoning training. Her latest attempt at conjuring and dominating a lowly monster failed so spectacularly the fiendish thing exploded all over her, leaving nothing but ooze! It ruined her lovely dress she had picked out for a night out after she was done with practice for the day. Not even the advanced cleansing magics she knew managed to save the white lace hems of her favorite dress. Her petticoat she could do without, but having her most beloved piece of clothing tinged a sickly yellow. When asked how she caught a cold in the middle of summer she would always swear it was due to the slime, that she must've gotten some of it in her mouth and swallowed it.
Of course Aini never told anyone what really happened. She couldn't tell them her first partially successful summoning conjured a slime-girl named Inia, that she had a lovely chat with before realizing she had forgotten to draw the submission runes properly. It allowed Inia to slowly, carefully melt part of her body and snake a tentacle made of her own bodily slime all the way behind the unlucky witch. While Aini had noticed Inia was diminishing in size for a while she attributed that to her only managing a temporary summoning.
When Aini turned to consult her notes she had finally noticed the slime's appendage; far too late to react with anything but a scream that Inia muffled by plunging that limb into Aini's gaping mouth.
The resulting struggle, the way Inia used her diminished body's surprising strength and wrestled Aini into an embrace as the slime disappeared into the witch took only mere minutes, though Aini could swear it took hours of breathless struggling against an unrelenting trickle of sapient sludge engulfing her innards. She was never truly certain how long it took as she passed out, deprived of oxygen and with a sense of being so wholly invaded she figured this thought had been her last.
Aini had awoken in a bed of the school's medical wing, dressed down and cleaned, though she continually coughed up yellow slime that evidently also clogged her sinuses, making her nose trickle a constant trail of yellow sludge. The nurse on staff had explained it was 'summoning sickness' and Aini didn't wish to argue that in any way. Admitting she made such a rookie mistake and got her body invaded by a slime was not an option.
So Aini did what any reasonable witch would: got herself out of the medical wing as quickly as possible and returned to her routine at Novella High. For the first day that plan even worked out, almost. Aini had to blow her nose almost constantly when she wasn't busy coughing up yellow slime. Her teacher got sick of her constantly dumping her sludge in tissues during their lectures and sent her on a mandatory 'quarantine' for a week.
Despite the contempt Aini felt at the teacher's demand — which was put into writing and signed by the head master's secretary — she had to admit that the constant oozing of her nose and mouth was concerning. Once the rookie witch arrived at her dorm room she decided to try a cleansing ritual she read about once.
As soon as that thought manifested Aini could feel her lips move on their own, her voice box twisting as something forced her to say "I wouldn't do that if I were you," in her own voice. But Aini couldn't even process those words, the sensation of having her voice used like that threw her into a coughing fit so bad she had to sit on her desk, in front of her dressing mirror.
The coughing fit passed eventually, leaving Aini's throat and lungs burning with a dull pain. She sat up slowly, leaning back with her eyes pressed shut before she lurched onto her desk, staring at herself in the mirror. What the witch's reflection showed made her sit up straighter. The sclera of Aini's left eye had yellowed completely; not just a yellowish white but a filthy, sludgy yellow, much like the slime-girl Inia's body.
The left half of Aini's lips curved into a sickly smile as she again her herself say "What, you didn't expect me to just vanish, right? You have only yourself to blame for letting me possess you."
While Aini still processed this development the voice that must be Inia continued, motioning with Aini's left hand as she mused "You have a pretty healthy body, I'm impressed. Didn't expect your gut to be so full of glitter though. You wanna talk about that?"
Aini, not sensing the motion in her left arm, instinctively lunged for it with her right and pressed it to the desk, relieved to realize she still felt her possessed limb. She shifted her attention back to the mirror, staring at her yellow eye, the one engulfed by Inia. Aini growled "What the hell are you doing?"
Inia replied with a chuckle, the sound full of mockery, "Making a point."
"What?!" Aini screamed, her body trembling. "You're making a point? How did I just let you possess me and my voice?!"
Inia laughed even harder, "You let yourself be possessed! I was merely the one who took the opportunity! If I hadn't been able to get in your throat I would've just jumped on you and possessed you fully. I'd figured a possession from within would be easier, but turns out you're somehow resistant to me. I can only exert control in part. Not sure how the hell your soul is doing that."
Aini felt something in her throat twitch; her lips managed to move for once but the result wasn't a voice but an awkward sound.
Inia cackled, and Aini felt a pressure she hadn't noticed before vanish from her throat. "Sorry," Inia said, "you can speak freely now."
The witch coughed a couple times, her voice sounding harsh but working well enough. "Why... why?"
"Well, for one," Inia said, "I don't want you to die. I'd lose my host then. And for another I, well, I think it'd be cool if you let me ride along for a while? You know, I'll be your inner voice and help you decide stuff you're unsure about, stuff like that. I can also do other stuff if you let me manifest slime appendages on you. Enhance your senses and stuff. I just... I don't wanna go back to the wilderness and I don't want to become a fugitive amongst humans."
The possessed half of Aini's face slackened, making her look human again as she leaned back, still staring at the mirror.
Inia whispered "You know you can just ask me questions now, I'll answer them truthfully. I'm trying to build some trust here."
Aini sighed, smacking her lips. Realizing they were coated with Inia's slime she quickly wiped them.
Aini wished she could just be rid of Inia, but that would involve admitting she messed up; meaning it was not an option. Not yet at least.
"So you're fine with riding along? That's it?" Aini asked, staring at herself in the mirror.
Inia used Aini's left shoulder to shrug and said "Sure, yeah. I may voice my opinion sometimes or mess around a bit, but I won't put you into harm's way. Slimey promise."
"Right," Aini muttered, flexing her left arm to regain control of it. "I guess that works for me. I do have some questions though. Like, is there a better way to talk? I don't want to stare at myself while addressing you, Inia."
The slime chuckled and sighed before placing Aini's left hand over her mouth. With a cough and a hack a large blob of slime shot out of Aini's unprepared throat and landed squarely on her desk. The blob of yellow ooze throbbed and twisted in on itself for a long while before turning into a miniscule version of Inia, smiling up at Aini smugly. "I'll just use this little copy of myself so we can talk face to face then."
If it weren't for the fact Aini just coughed this little blob that turned into Inia up, she'd have thought it to be cute. There was even some glitter swimming in the slime that made up mini Inia's body.
"Now then," mini Inia said, performing a courtsey, "Let's talk. What do you wish to ask, dear host of mine?"
"Well," Aini started, taken aback by the title of 'host'. "