I had a (very unrealistic) idea come to mind where it would be possible to corrupt Kalysea by deliberately giving her corrupted/tainted artifacts and not warning her about them, similar to how we can corrupt Viv. There's not any currently on her list of accepted items, but I'm thinking of things like the blight tendril, girthy rod, and maybe something like the ring of temptation? (Which is on her list ATM) and really just handing her anything that's tainted or expressly corrupts the player when used.
It wouldn't produce any immediate effects after she goes and gets her cockring made, but as travel scenes procced it would sometimes trigger a special scene that was headed with a "This time... Something seems different..." text to jolt players out of travel-zombie states. Then it'd be a scene where Kalysea has some particularly rough or dominating sex with someone that Kalysea comes out looking a little bit different, something like she's a bit taller/bigger, has a bigger cock, bigger breasts, etc. Something that she doesn't immediately notice.
Then as more and more of these scenes begin to proc Kalysea begins to slowly change more and more before eventually culminating in a big scene where she's trying to masturbate or get us to jack her off and she starts to cum her soul out... Only to have it get stuck behind her cockring. So as she's bucking and writhing to try and cum, not knowing that it's going to turn her into a demon, she's also transforming/growing dramatically as she thrashes around. At this point she may or may not realize what's going on with her soul and start panicking that she can't stop, or she might just embrace the change because it feels good or the tainted cockring screwed her mind up.
The climax (pun acknowledged) of the scene would be her corrupt cockring shattering as she finally cums her soul out, leading to her potentially gaining a hyper cock or multiple cocks (or multiple hyper cocks) through her desperate want to finally cum. Finally freed of her inability to cum, she looks down at herself to realize that she's become a sort of semi-giantess/giantess, like how the behemoth is 14 feet tall. Maybe have her average out at like 15-16 feet?
At this point the player would be left with one of a few options, either sending her to the alarune's care alongside the behemoth (and potentially unlocking more content for him or both of them at once) or simply allowing her to keep traveling alongside the group, opening up the possibility that she might spread corruption at some point, though I don't have a ton of ideas on how that might happen.
TL;DR: It would be neat to have the option to corrupt her into being a genderflipped version of the behemoth.