Mutual co-dependent attraction is quite an apt way of putting it.
I like all of the ideas really. Perhaps, we could try and see what develops naturally? I normally despise doing so, especially when given the opportunity to potentially think it out ahead of it. As for ensuring quality and meaningful adaptations to be involved with their building character arcs, if you've given three possibilities, all of which I adore, then I imagine that it'd be an enriching process for both characters as we go.
Namely:
Kaden and Satsuki have an on again off again moment to moment sort of dynamic where she comically, somehow, misses or miss-constructs events such that she doesn't pin the evil card on Kaden as early as everyone else does. Sort of like that person who subconsciously forgives/makes up for/excuses or just blanks on their partner's short comings, just that their connection is a little looser.
or: As they grow closer and she figures Kaden out, the evil spirit arc comes to a close and Kaden comes face to face with the opportunity to steal Satsuki's own soul to further the evil spirit's plan... But gives it back. Its possible that there are redeemable qualities of the soulless Kaden and the drawn-in Satsuki may feel it morally correct to try and guide him toward a path in which he could do more good than evil. A redemption of sorts. Potentially, Kaden could try his best to fight his inescapable lusts for this amazing girl who is seemingly doing so much for him. And yet, despite the chance to take back his soul and have the consensual human feeling love, Kaden refuses. Going so far as to probably ripping the card that houses that part of him, as he fears it ever returning to him at all.
or: The building tension of corruption and moral degradation of their interactions starts eating away at her. They need each other that way, but as a protagonist, she can't keep meeting him in secret without telling the others. Lying isn't sitting well with her and in her wayward attempts of making this situation right- She loses one last time. (Or she barters her soul off in her own heroine attempt to do some good) Leaving her one to one in the same situation as Kaden. And the relief of the guilt can do a lot for her, as it has for him. Either allowing her to indulge in the physical pleasures of their relationship guilt-free, or allowing her to really see how much of their empty relationship relied on that decadent unobtainable or socially/morally incorrect bitter sweetness to have any value which could lead her to ending the relationship on a constructive note.
Adapted versions of your ideas, or what I thought of them. Thoughts? (Clearly not one for one rewrites, of course.)
Edit: Of course, these are only applicable if we go for this.
Edit2: Yeah, after your post that order should go
Satsuki
Kiro
Galan
Satsuki
Kiro
Galan.