Reasons:

The first reason:

(The weakest reason)

Just like other candidates, he is absent during the game, which might imply that they are doing something. The reason this evidence is weak is because toriel said that he's in an university.

The second reason:

(A weak, but also acceptable reason)

He has horns, which might imply he is the knight, since they both have horns.

The third reason:

(An OK evidence)

Asriel's saber in undertale look almost exactly like the knight's sword.

The fourth reason:

(The strong evidence)

There is a theory that says that asgore might be working with the knight because of a scene i chapter 4. If that is true, knowing it is plausible, it would make even more sense for a father to work with his son.

The fifth reason:

(The strongest evidence)

The strongest reason doesn't come from deltarune itself, neither undertale, but actually from earthbound. In earthbound, one of the main villains is the protagonist's supposedly dead brother that doesn't appear during almost the whole game. Do you know who fits in that description? Asriel.

Extra reasons:

(Reasons after the post being edited. If it is not by me, i'll put who said each thing)

1: The Knight's attacks are very close to Asriel's attacks in Undertale;

2: It would explain why kris deals less damage to the knight, why the knight deals less damage to kris and instead of swooning kris like the other members of the party, they just down kris;

3: both of Asriel's final forms in Undertale seem way too close to Knight's look.

Conclusion:

(The conclusion of this theory)

Well... I am happy for being probably the first person to bring this theory and give points to it. Also, after a quick look on that, it is better than dess-knight or carol-knight.