Thank you so much for your suggestions!
Throughout my previous deliberations for the story, I've considered these options alongside other available ones. Unfortunately, they didn't solve the problems Kai presented as a love interest, and weren't cohesive with the narrative.
There's a certain motif I want the relationships of IseKai to have, and Kai's character doesn't have it. His route stood outside the current logic and characterization of the world, and I have no desire to restructure the foundation of the world to make a tragic romance that wouldn't satisfy players for the amount of effort and time they put in.
For those reasons Kai is better as a side character than a love interest. :)