Thanks. Does the MC know that she is causing all this chaos just so the MC can go on arbitrary quests to lvl up? So that they can be plucked later when they get juicy and fat? And the MC is ok with this? Or does the MC somehow believe that the power of love and friendship can make Kas give up her obsession and cure her, thus they will continue to oppose Kas and solve the problems in the world while still loving her, despite knowing that she is causing all this mayhem just to fatten up the MC.... God, even writing this is making my head spin.
My opinion after reading your analysis is that this seems incredibly convoluted and flimsy. For one, it cheapens the plot and lowers the stakes drastically. "Ha! You were just on a wild goose chase all along." "My machinations lay undetected for years..."
If you go full romance with Kasyrra then yes you can get her to tell the champion her plans and where they fit into them. As far as what the MC believes well that's up to you as the player assuming the role of the MC.
The most straightforward way I can see it is that she's playing a big game of corruption and hedonism because that's in her nature as a demon and no one around can do anything to stop her and she knows it. She's also smart enough to lead the MC through the right hoops in her game to get her what she wants over and above pure chaos and sex while she's having some fun on the side. Two birds with one stone so to speak, plus she likely has some other pet projects going on anyways since she also said she as a researcher of sorts, it wouldn't surprise me if she had a backup plan or six on standby just in case.
The champion is the first person she saw in Savarra and their soul was already goofed even though she shielded them from the explosion so she saw a chance right away and used it to start setting things in motion. It's pretty pragmatic of her and it gace her the time to explore the new world and set up some other schemes i.e. getting her daughters over as well. She gets to have fun (literally) screwing the world and anyone she wants while at the same time she fattens up your soul until its ripe for the taking, sounds like a win win.
As far as saving her and all that jazz, well that's sort of up to you and whatever your own headcannon decides about her, whether it's all bullshit or you want to redeem her or help her or you just want some dick.
It's a bit convoluted sure but this is primarily a porn game and I kinda like that some of the reasoning is left up to you to decide. Even though Kasyrra never chose to become a demon she's embraced it now so you can see it as she's some sort of addict and try to help her if you like her or kill the evil demon because you're a chaste and pure traditional hero. That's all up to you.
Personally I like Kasyrra and think she's one of the better characters so I tend to take the time to explore options with her and I also like the tongue in cheek vibe of meeting up with her on the sly and hanging out and eating cookies and playing video games or whatever while you talk shop about world influencing plans and critiquing each other before going back to see who's going to win your little game that just so happens to concern the fate of the entire world. Kind of like putting pause on the apocalypse quickly to make a Mcdonald's run then returning to the battlefield between the forces of good and evil after you had your nap, picked up your cousin from school, and caught up on your Netflix series. It's funny and it goes against the "quick only you can save the world from ending" rpg tropes so why not.
Oh thats nice...
Then why is he present in this scene?
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I checked the beginning out and I think this is a typo or sorts since you have two options right after the fight with Tollus. You can either comfort Cait which leads to her trying to get into the portal and failing and then leaving back to town while you check out the portal again and Kas shows up when Cait's gone, or you chase after Tollus' crew.
If you chase them they still teleport away and you tell Cait everything's going to be okay up on a hill, then she says she's going back to town and asks to be alone for a while and you go back alone to check out the temple just in time for Kas to show up. IDK why Cait is in that scene because the one right before it strongly implies that you two split up and your there alone but I doubt it'll get changed even if someone brings it up on the CoC2 forum.