If there's one thing I've learned, Savin makes kas as subtle as a oil tanker through a fishing shack.
While there are indeed no clear indications of this being the case, it's a really good hypothesis. It fits very well with everything and solves some of the worst problems with the "Calise is Kas" theory.
While not having a weakness, her lowest resistance is Mental damage. Corrupted Libido makes tease damage a poor choice unless you went full tease build. She also has Blade Block, which nullifies melee attacks for three turns with a recharge of 8, giving you five turns in between blocks.
If you yourself can't deal with the Faceless, try healing Calise and purging her starting status effects. If you can keep her up during the fight she should be able to handle things pretty well.
I was obvious to me that Kas is Calise from the moment the moment I first saw her because of the option to ask Kas about her human form/past. I even left a comment about figuring out Kas's secret identity on the post were Calise Quest 1 was added, It's even more obvious once Freja ages into a toddler and it says she must've gotten her tan skin tone from Kas's human form.