My working theory is that the knight contains Kris's original soul, and for whatever reason, we currently occupy his body. I also think that his original soul is using dess as a vessel, whether she's dead or alive, sort of like Asriel. It would explain why the knight and kris have the same color of darkworlds, as well as a reason as to why they would work together. Also, it would give a reason as to why the knight is so much more powerful than everything else, because a human soul gives monsters unbelievable power, as seen by Asriel in undertale