>>717886354That's easy, the thing that we fight simply isn't the Knight's true form. It is some kind of puppet or projection which has been made in the image of a Holiday.
We can assume this because of two things:
First, we know that the Knight is a Lightner, but the Knight's form is impossible for a Lightner to possess.
Second, the Knight wears a helmet that can be removed.
The Knight's body is amorphous and liquid, and seems to be made of Darkness just like the Titan's. In fact it even turns into static when attacked, just like Titan Spawn. But we know that the Knight is a Lightner, who can move in the Light World enough to drag Undyne around and make Dark Fountains. These two things can only be true at the same time if we assume the Knight made of Darkness is not exactly the same as the Lightner who is the Knight, and in fact given how the DW Knight's body seems to be 100% Darkness, it is likely that they are not the same at all.
Therefore it doesn't make sense to assume that the Knight's DW appearance speaks to their LW appearance, because the former seems to be completely artificial.
Secondly, we know from the "Think of the Knight" cutscene that the Knight wears a helmet that can be removed. However in game we can clearly see that the Knight we fight is not wearing a helmet. The angular thing on top of their torso is just a part of their body, and acts as an eye, mouth, and head at different times. There's no "helmet", just body mass.
So then what's wearing the helmet? Connecting to the first point, I think it is the Knight's LW body. But why would the Lightner Knight need to wear a helmet? I believe it is to control the DW Knight's body which, if my first point is correct, is not the Lightner's real body.
If all that is correct, and the Knight is Asriel, then one need only assume it looks like a Holiday because Asriel made it to look like Dess, maybe because it's inspired by her in some way or maybe because he's using it to find her.