I like to think a Roaring-related demon or Gaster or something is the Knight and they’re also possessing Kris, since based on some stuff it at least seems possible (Kris studied occult so could’ve done something that turned into a big mistake, they’re freaked out by whatever is in the locked bunker in the woods and iirc it has a slowed down Gaster-related noise, they have glowing red eyes and a demon smile, the fact they can rip out their soul in the first place). Some people think it’s Kris themself struggling for control, though, however it seems to be implied if not made somewhat clear that they care for their friends and are mischievous at worst, but I guess we should just wait for more info.
I hope that, if it is Kris themself doing the soul-ripping and Dark World creating, that they aren’t doing it for a malicious reason, since it seems to contradict how their personality seems…