It could just be stylistic choice, but when Kris' soul begins to shine against the Knight's attack in Chapter 4, it first glows pure white (like a monster soul) and then has a glowing white outline.
As we know, humans can absorb the souls of Boss Monsters (Like Asgore, Toriel and
Asriel) which enables them to do things that normal humans can't, like pass the barrier (in Undertale), and possibly seal Dark Worlds.
Pure schizo theory, I think that Kris' soul might just be a combination of a human and a monster soul. Possibly even Asriel's soul.
