Notes/disclaimers:
I have no idea how a Saix/Xion ship is canonically possible; this is my tentative guess as to how a relationship between them might look if it were to happen, written under protest because my friend is evil.
Beta'd by the wonderful a-perfect-nobody. Any errors or problems that you find are purely my fault for messing with their edits.
Warnings: non-explicit rape/dubcon and abuse
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If he had a heart, Saix might feel guilty about using Xion like this. Then again, a heart might allow him to understand why he does it at all. But he doesn't really want to understand, and that makes it easier to handle the whole situation. He has no interest in Xion; the duplicate had been promising once, but already it has begun to show its uselessness. Now that Xion is starting to break down, the best course of action is to get rid of it so it can't interfere any more. And yet Saix keeps coming back to it, or allowing it to return to him – he isn't sure what's going on any more, and it gives him a headache to try and make sense of it all.
It might be revenge that motivates him. Axel's betrayal – how can Lea replace Isa with a puppet that isn't even a Nobody? – tastes of bitter ashes that he has to force down whenever Saix sees him with Roxas and Xion. Thinking of how his best friend is getting closer to those two while drifting ever further from himself, Saix almost believes that he can still feel hurt and that makes start another feeling burn in his heartless chest: Rage.
There's nothing he can do about it, but 'nothing' would never quench his rage. So he takes it out on Xion, in cuts and bruises and bite marks deep enough to bleed, and the puppet never complains. It should suffer more than him, know how useless it is and that it never will be more than a toy. A toy to punish for the loss of his friend. One day, this little toy will regret that they ever met.
Perhaps it even thinks that Saix feels anything other than contempt for it; it is certainly naïve and foolish enough to believe such a ridiculous notion. But that only makes Xion easier for Saix to control.
It's at times like this that Saix does not miss having a heart. As a Nobody, he can free all cruelty within him. Only Axel knows him well enough that he might guess at the despair that this thought can cause Saix; only Axel knows Saix as more than the cold Nobody all the others see, and to lose that… He may be angry at Axel, too, but Xion is far better a puppet to play with.
The other members refer to Xion as 'she,' as though it is a real girl – as real as a Nobody can be. Saix has known it more intimately than any of them, and has seen nothing but blank darkness where its face should be. There is significance in that, he's sure, if he's interested in finding it.
Saix ignores it. No one bothers trying to understand a mere toy.
The sheets on his bed rustle as Xion shifts, finally waking up. As always, Xion pauses at the door as it heads out, like there's something it wants to say. Saix glares at it. He never allows Xion to speak in the mornings after; he doesn't want to know what it might have to say. It avoids looking into Saix' eyes, uncertain. It has learned, he thinks with satisfaction. He remembers how long it took to gain proper control over it.
When Xion will finally break down completely, Saix knows that he will not miss it. But he is drawn to it nonetheless, and doubts he'll stop playing with it before then. Sometimes he wonders if this game he plays really has anything to do with his original purpose any more. Revenge may have started this, but now he finds playing with Xion, seeing what might make it break, entertaining above all else.
Xion leaves his room in quiet, hesitant steps. Saix, knowing that it is still far from destroyed, smiles.