That's the point of a visual novel though, to have choices and to shape the story how you want. That's been the way for a lot of choice based games, even non-porn games like Detroit become human, heavy rain, and tell-tale games for example. The canon storyline, and the characters are based on the choices you make, and other character besides player characters can change in some games depending on choices.
So yes, Kath is a naive, shallow, and unsympathetic person in that last scene, depending on choices, or she's a thoughtful, caring, and loving partner, depending on choice. That's the whole point from my point of view at least. In my save she didn't let them get anywhere with her, so she was the latter from my point of view. I've always strongly disliked the argument, especially in NTR/sharing games or games with optional content regarding those themes, where people argue "Well if she can possibly cheat on you it 'RuINs heR chAraCTeR'. Even though, it's literally a choice based visual novel, where you're meant to shape the story and the characters based on choice, and your own playthrough is your own canon. It's a really brain dead argument.
Sorry for the rant, that's all from me.