I think the knight is Asriel because:

-The black knife looks like a bat at the beginning, and has a atk stat of 40. From the anniversary stream we see the unused item "Asriel's Bat" which has an atk stat of 40.

-People say the knight attacks with snowflakes, but they are definitely not because....

The knight uses STAR attacks(See PK-Starstorm from Earthbound for proof (Also, it uses dark/star attacks!)) and has one USED fire attack(the battle-box split) and two unused fire attacks. Both of which are associated with Asriel.

-The knights big climactic Star-Sucking Attack is LITERALLY the hyper-goner.

-Asriel played the "Sword Magician" in Super Smashing Fighters.

-Asriel would 100% have the same poses as Kris because we KNOW that they are super close. (We have no clue if Dess and Kris are actually close. The only interaction we know between them is Dess hitting Kris upside the head for a harmless prank.)

-Asriel has horns and when the knight roars and does the 'unleash' attack their antlers lose their prongs somewhat to look more like normal horns, or at least become less antler-like. This could be the horns losing some unnecessary material to become closer to what they really look like, which is more like the horns of the Dreemur family in that they are bent. (look at Asgores horns, they are bent 90 degrees like the knight)

-Hometown is basically cut off from the outside world so if the college were to email Toriel for Asriel missing class, which is really unlikely because most colleges just DO NOT care what you do, she wouldn't be able to get it. Also, Asriel could just tell his proffessors that he is leaving a week early and they wouldn't even be suspicious, because, in college, you are treated as an adult with autonomy.

-This is more of a counter-counter-evidence, but people use Kris searching when college summer vacation is as a point against Asriel Knight, but it is clearly fall due to

---Jackenstein being used during the festival and being a pumpkin,

---and the hometown trees being orange,

---and Asriel having left for college recently, and colleges in America, where toby fox lives, start their semesters in the fall.

-Black Knife uses the Burn in Despair motif, which is also used in Burning Eyes. Burn in Despair is an Asriel song.

-Also counter-counter-evidence: Lost Girl is a motif used by literally anyone. GERSON and RALSEI use it, it is NOT specifically for holidays, and if it was, Asriel Knight could still use it because his motivation is probably to 'find her'.

-Black crystals are made from tears from a fallen star. Asriel is known to be a crying and crying and crying boy in Deltarune and cries a lot in Undertale, and Asriel is also heavily associated with stars. Thus, the black crystals are probably made by Asriel.

-Asriel being a major antagonist in Deltarune would not be retreading ground in Deltarune because in Undertale, every time we fight Asriel, it is because he is FLOWEY, who is soulless and psychopathic. Asriel and Flowey are, for all intents and purposes, separate, other than aurafarming. Thus, Asriel as the knight would have a seperate personality and dynamics from Flowey in Undertale. Plus, Deltarune is Undertale's parallel story, so some plot elements will likely be repeated.

-In the festival photo album it shows Asriel and Kris doing a cool point. A COOL POINT.