Deltarune Chapter 4 revealed something huge right under our noses: Dess was a Human.
In December Holiday's room, as shown in Chapter 4. There exists a closet. In this closet, is a huge pile of shoes and an even bigger pile of Red Heart-shaped pillows.
These shoes, interestingly, are drawn as normal, plantigrade shoes like those in real life, fit for us of the human race.
Deltarune has thus far gone to great lengths to tell us that monsters like the Holidays, with hooves, do not wear human-style shoes and instead wear ones specially fit for hooves (assuming they even bother, which Noelle doesn't).
We even see both Rudy and Carol wearing these shoes, as shown by a downward bump at the back unlike Noelle's naked hooves, and Rudy's shoes going way too far up his knees to be hooves.
A pair of shoes noted as being fitted for hooves are found in Dess's room, but unshown and in a pile of things that aren't all hers. All the shoes that are actually shown in Dess's room; a large pile in her closet, and a pair of roller skates (where proper balance is especially important) are plantigrade shoes made for humans.
This would be strong enough evidence, given the circumstances, and yet right next to the shoes in the closet is a particularly large pile of Red Heart pillows. Carol is noted as collecting these, and they are also described as "Human Soul Pillows. (depending on who you ask.)"
Since Carol is heavily suggested to be part of an investigative conspiracy looking into Dess's disappearance and/or trying to get her back, it's very likely that this minor obsession with Human soul pillows is related to Dess, as she herself may have owned a soul of red coloring. (Or perhaps more of a Vermillion, based on her shoe and guitar color.)
If Dess were human, as it very strongly seems, then it would completely recontextualize her and Kris's relationship. Kris's narrative subtly revolves around their sense of being an outcast as the only human in a town of monsters, but it's never suggested that that status has always been the case. If Dess were also human, it's likely the two would be extremely close, and Dess's disappearance would deeply affect Kris, as they began to feel alone in a way they couldn't even comprehend before. It's even possible that the two were like siblings, having been adopted from the same orphanage.
Even Undertale, with its small hints towards the Holiday Family's existence, makes sense. Humans are banned from existing in the Underground, but the game and its surrounding media go to great lengths to never suggest that the sister Noelle has in this universe is the same Dess or a reindeer monster. With Sans deliberately avoiding describing her in any way despite describing Noelle, and her not appearing alongside Noelle in the Anniversary Livestream.
If the Holidays are prone to adoption in every universe, like the Dreemurrs also seem to be, they may've adopted an older sister for Noelle that could be any monster subspecies.
Dess's only established appearance thus far is in a flashback depicted by Tenna, where she's shown as a silhouette sitting on the floor, vaguely the size of Kris in the current day. And she seems to have antlers, which would seem to debunk this theory, until you notice that Kris also has horns.
Kris used to wear a "devil horn" headband when they were a child in an effort to fit in with their family of horned goat monsters, so it's very possible that Dess wore a similar headband with reindeer antlers. (And ears if you don't believe the spikes on the side of her head could conceivably be hair.)
While I don't agree with it, as its issues are numerous, even the Dess!Knight theory fits strikingly well into this narrative, as Chapter 4 depicts a short scene where the Knight is capable of taking off their helmet to reveal some kind of hidden face, suggesting the antlers are fake/removable, just like an antlered headband.
(Individual Panels under the cut)
