1.the shape of the body

aside from the antlers the knight has a skeletal frame which when yelling literally shows a ribcage now dess has been gone for a long time, and if we take a look in her room we can find many interesting things, like rations shoes fit for hooves and other stolen items which i dont know how you see it but rations and shoes show that dess was an outdoor person, because rationing food is something people do during long camping trips, and shoes, she is a reindeer monster she doesnt need them but aside from wanting to look "cool i would say it is for better grip on the ground so what could dess being an outdoor person mean, well since she disappeared when noelle was young (which was years ago) this offers the chance that dess mightve became malnourished and corrupted by a dark fountain which would explain the skeletal figure during the knight's yell

2.the text when the knight slashes tenna's arms

to simply put it the text is is red and the colors we can associate dess by are the colors in her room and her guitar the colors in her room being a glow in the dark type green and some red the guitar is also red (i know it's orange but it is literally called a red guitar) and that red bit of numbers is the only time something like this appears and we know from when noelle joins our party in chatper 2 the color of damaging something text doesnt always match the main color pallet since noelle has a mainly white color pallet but here damaging numbers are yellow the same could be for dess so yeah we can associate red with dess

3.the magic attacks

now we are going into more crack theory territory but here is the theory that crystal attacks are corrupted ice magic i mean they resemble snowflakes and they are the only attacks i can list that fade away unlike most which move off screen, kinda as if those crystals were melting

4.queen's recording and peon choice

now i'd consider this an actual crack theory but we know queen has a recording of the knight forming the fountain but she doesnt know who the knight is (probably since the knight's face was covered) but she still recognized that the knight is deer shaped and i'll explain why i think that this was imagine a computer haveing to identify i person (toby fox for example) under normal circumstances the computer could say yeah that is toby fox but if the face is covered more than likely the computer wont know it's toby but it'll still recognize it's a human so the theory is queen saw the knight didnt recognize them since the face was covered but still recognized that it is a reindeer monster so that is why she chose noelle

5.spamton-dess/secret boss-knight connection

so now this is a real double crack theory the unused dialogue for equipping one of spamton's armors onto noelle, noelle would say that it feels like guitar strings so there is a connection between spamton and dess and since every secret boss has a shadow crystal most likely from the knight it could mean that spamton has connections to dess and since he is a secret boss and has connections to the knight the knight and dess are connected (since y'know they may be the same person)

6.the pose

in chapter 4 staircase squence when they shut the door before susie could get there
the knight has a pose that appears to be laid back and nonchalant which also appears to resemble someone leaning on a way the knight's arms are even crossed which further proves that they are doing the "leaning against a wall" pose even though they are only leaning on air
there are little amounts of characters in deltarune who would pose this way (i mean out of all of the characters in the game the only other one who does that pose is susie during chapter 1 when you ride the elevator for the first time right before she "actually" joins the party)
so with all that evidence i would be most confident in concluding that the dess-knight theory has one more bit of evidence now since all the behavioral patterns of dess which were confirmed and the ones we can speculate due to the stuff located in her room would show her to be a nonchalant at times person who would do that type of pose when waiting
and this could also be classified as evidence against the asriel-knight theory since he is described too much of a dare I say goodie-two-shoes of a person to even think about doing that type of pose and yes i know i stated earlier that i believe that the knight is a corrupted version of dess but this doesnt go against my point because it is possible that some behavioral patterns stayed along

7. the height

in chapter 2 after closing the fountain we get the text a tall person could fir in her and now i got info that we should base it off ralsei being 176 cm tall so based off that 1 pixel is about 4,5 cm tall so to make it more accurate ralsei would be close to 175,5 cm tall (horns excluded) and noelle would be close to 184,5 cm (antlers excluded)
then going off the added height by age it would be asriel is close to 180,5 while dess is around 186,5 (horns and antlers excluded) so now since these are fictional characters i will use some more in world data for horn and antler height
with this height logic ralsei's horns would be around 31,5 cm and to add for the growth process for asriel it would be 3 cm a year so 6 cm added meaning 37,5 cm added later to the height of asriel
as for dess's i am this time gonna base it off the height of rudy's antlers since the height it may grow could be higher than a normal reindeer (and in the tenna flashback it is seen thather antlers are more like rudy's), they would be around 40,5 cm but since femal reindeer antlers are shorter it would be safe to assume that they would be somewhere in the middle/upper middle of length like rudy's since for males it's 30-70 cm and carol has 36 cm which is longer than a female reindeer should have so about 32,5 cm added to dess
so in short asriel is
180,5+37,5=218
and dess is
186,5+32,5=219
which the relative height now is closer but dess still is taller than asriel by 1 centimeter

she wore shoes too so she might even be 2,5 cm taller than asriel if we include that because asriel more than likey didnt ever wear shoes rudy could also usually have longer antlers but his sickness may be causing his antlers to grow weaker so dess might have full length and be 3,5 cm taller than asriel without her usual shoes and then 5,5 cm taller with shoes
also since toriel says that kris will be off to college soon this could mean that they are 17 causing that the height difference between asriel and dess would be either 4 cm or 6,5 cm (dess not wearing her shoes fit for hooves), or 6,5 or 8,5 cm taller than asriel (with her wearing shoes fit for hooves)