Here are my headcanons. If it's not confirmed and it ends up being drastically different from what I drew, I'll probably keep the design.

Headcanons:

1. The Roaring Knight is a wendigo. You can see how his head turns into a deer's and his armor turns into bones when he roar

2. The Knight is December. The game touches on many important themes:

A) The Dark Ones can't escape their world into the real one. This is definitely emphasized.
B) Fate, which cannot be changed, doom. An omen.
C) The imbalance in the world caused by the increase in the number of fountains.
D) Kris's connection, presumably to the Knight.
D) The Holiday family's mother's connection to the Knight, to the catacombs, and the phrase "YOU are always welcome here"—she said this when she kicked Susie and Kris out of the house. But who exactly did she say this to? Kris? The Soul?
E) Death and Rebirth

This is only a small part, but these significant points are enough to consider Des's connection to this, as we are constantly reminded of her and mentioned throughout the game. If she weren't an important character, they wouldn't have tried to immerse us so deeply in her lore and give us the opportunity to explore so much.

I assume the Knight is a Dark version of the dead December, who is being resurrected. Remember what happened to the old man when Susie tried to bring him back? Nothing good.
The Fountains create an imbalance; they can provide an opportunity for the two worlds to unite, but they can also instill horror, the coming of the Titans. Perhaps the Knight wants to return to reality, to regain his life.

3. The soul inside Kris obviously doesn't belong to them. I have an idea that she's been prepared for December, and Kris ultimately leads us to her, to the Knight.

4. Noelle has been deeply distressed by the loss of her sister. Her attraction to the macabre can be explained by curiosity, a way to connect with her sister's interests, but also by processing trauma. Someone who has lost a loved one and suffered trauma wouldn't go to a cemetery to have fun. A coward who's afraid of horror movies wouldn't force herself to watch them. All these oddities are unconscious processing of trauma. Over time, Noelle began to derive satisfaction from watching horror movies; she continues to be drawn to anything threatening, dangerous, and creepy. Kris also plays a role.

Thus, Noelle internally comes to terms with the loss of her sister, feeling closer to her rather than afraid.

And now for the cute facts!!!

5. The Holiday family are reindeer. This headpiece was born from the idea that their entire interior, interests, and even names are tied to Christmas. Also, in their family, both girls and boys have long antlers, which is typical of reindeer.

6. They can change fur color depending on the season. I imagine Des with dark fur, or gray, or lighter. She literally looks to me like a character with cool or dark tones. Noelle is a red reindeer (which is also possible for reindeer), and in the Dark World, I imagine her with light or dull fur. She controls snow and frost, and the snowy Noelle has light tones.
Overall, they both resemble the two seasons, winter and spring, which adds more charisma and recognizability to the design.

7. Christmas customs and legends. If Noelle went camping, she'd spend the night around the campfire telling scary stories about the evil Krampus who steals children.

8. They eat moss, and that's okay. They're deer, after all. No discussion. Chris is on the same wavelength.

That's how much can be written about a character who's literally dead and never even shown—

Maybe I'll write something else later.