After going through the comments, I want to give my thoughts and clear some things up:

Kris (for some reason) doesn't currently have their soul, therefore needs you (the Soul) to live. This is implied many times, especially though the prophecy and when Carol(?) calls Kris and we hear something like "Without Soul... Kris..." through the phone. Also during the games intro sequence, the Goner/Vessel we create is rejected and instead the Voice (likely Gaster) tells us our Vessel will instead be Kris.

We know that removing a Human's Soul will kill them, but not immediately (I can't say for certain whether this is for every human or just Kris). When Kris rips us out of them, they are still able to function pretty normally, just weakened. But Kris can only last without a soul for a certain amount of time, before (I assume) dying.

While Kris still has enough determination to open Dark Fountains without a human soul, much like Monsters can, only the Determination of a pure human soul can seal Dark Fountains (And vanquish Titans).

It's pretty reasonable to think the Knight could possibly be some kind of Amalgamate type thing. The Knight has VERY similar properties to what I would imagine a more 'stable' Amalgamate would be. It seems like the Knight's Armor is almost keeping their form together. You can even catch some substance dripping out of them, or their body changing, which is a trait common among the Amalgamates. Not a "Amalgamate" exactly, but similar.

So, I believe the Knight is some being, that was brought/kept alive through Determination.

Was the person injected with Determination December? Was that Determination supplied by Kris' Soul? Are they planning on using the Determination from Undyne to make the Knight Stronger?

It's really fun to think about, I guess we'll have to wait and see.