Kris is wracked by guilt and self-loathing over Dess's disappearance and being human and thinks they are forever defined by it, never able to grow past it and be a person worth loving
This is why they're okay with leading Susie on to fulfill the Knight's plan (which probably ends with Kris dying let's be real) because "If I'm already a demon anyway it's OK if I do bad things and if Susie hates me in the end, it'll all be worth it"
On one route you and Susie and Ralsei will snap them out of it which will help finally break the prophecy et cetera
On another route we will let Kris indulge in these thoughts and the destructive behaviors that led to Dess's disappearance until they succumb to them and awaken as a True Demon, this time in a "If I'm already a demon anyway it's OK for me to have Fun" fashion, mirroring how Susie acted out because she already thought of herself as a Bad Guy

This will be how Toby implements Demon King Odie-o (Kris is a dog)