Kate, after update 0.50. It's when I'll start throwing polls for side character content again, so it depends how well she performs.
Mizu is unlikely to be continued. That route got really dark even for me and I find it unlikely there's many people who sticked with it. I only kept it since it was already made. It was that or throwing it in the bin. Not saying she's not getting anything 100% as update 0.42 is bringing more necromancy content and this route would fit in there (depending on your choices).
Was meant to be 0.40 but I got delayed with technical stuff.