Please do mind that this contains very dubious consent, folks. Disassociative identities are tricky things to categorise when it comes to consent, but this is not a happy sex fic in the slightest.
Spoilers up to trial 3.
Original prompt from the kink meme: "Celes and Ishida have a sexual encounter before the murders of Chapter 3. A "guest appearance" from Ishimaru is welcome."
In some ways she knew it had to happen, the way he'd been acting, and it does. It doesn't surprise her at all when he appears at her room's door with that strange paleness and the forced anger in his expression. They're alone together and it only takes a flirtatious smirk from her - no words at all - and he reaches out and touches her face and kisses her, and doesn't stop; keeps kissing her, face and neck. It's kind of strange but kind of nice to be pampered so surely, so she lets it happen and his arms are stroking down her back and she tries to do the same thing, and then he's awkwardly touching her breast through her dress and she freezes because no-one except her ever has.
She's kissed men, sure - a pretty girl winning everything in a casino can't resist smooching the nearest shmuck. But the sensation is sort of exciting, so she lets that happen too, and then he rolls her over onto her bed and he's fumbling at himself in his uniform and it occurs to her that this is about to happen. The sex thing, they're going to do it. She forces her heartbeat to slow. She is Celestia Ludenberg, and nothing can break her poker face.
He reaches for her skirt, so uncharacteristically nonverbal that it's refreshing. Then stops, and looks at her with concern - something must have shown on her face, something that wasn't apathy or ecstacy - and says "You're OK, right? Because I want to - I wanna do this. God, I wanna do this with you."
Ishida swears in twenty colours of the rainbow but he won't even say "sex" when it's happening? But then again, neither will she. "Of course." So it's happening.
He touches her lightly beneath her underwear. His fingers are warm, and suddenly one's inside her, now two, and now something much larger and more forceful than that. It's too much, and it hurts, she's small and he's big. But it's going to start feeling good; it's supposed to. So let it feel good.
She tries wholeheartedly to enjoy herself, and he's earnestly pushing, pushing, with hot breaths and an impenetrable expression - movement in her and in him and in both of them and she feels weak, grips onto his shoulders. And suddenly this is beginning to feel right, like everything's falling into - into place, and she breathes in short pants, and he does too, and it's wet and it's hot and she can feel it coming, that moment inside her, so different from her delicate hands in privacy.
But she sees it coming when it goes wrong. Just at the wrong moment, as his breath shudders and his eyes half close and he looks terribly ugly and she sees that moment of his eyes unfocusing for just a split second and she should have known.
He looks at her, and he stares, and he finishes, hot and wet in her. Christ, they didn't even think about protection, did they? But it's the wrong boy, and everything's wrong, and his face turns scarlet and he makes some sort of awful strangled noise of despair and pulls out, away from her, pushing her hard back into the bed as he stands upright and shakes.
It's cold and she's exposed and ashamed and feels like a little girl, feels like crying.
He pulls up his trousers and looks at her and she doesn't say anything because if she opens her mouth now, something in her is going to break.
He checks himself over and says, quietly, unsteadily, "I should go."
And she nods. And he leaves.
She pulls the coverlet over her, and allows herself to cry for a moment. Not for herself, but for Ishimaru, who is as surely now gone as he can possibly be.
With this in mind, she doesn't feel remorse as the lights go out and she starts planning his death.