DESS IS THE EARTH
So, an issue with all the not-Dess Knight theories is this:
If the Knight is not Dess, what story role could Dess even have? Is she just a helpless damsel in distress?
So here is an alternate idea of what Dess’s role in the story could be that is actually satisfying.
Dess is the EARTH.
It’s said in a few places that the fountains are specifically made by driving something sharp into the earth.
The KNIGHT that pulls the Fountains from the Earth.
Holy Fountains, whose shadows are creating a new world...
The Knight
The Roaring Knight...
Today, It Deigned To Create This World
Reaching Its Long Hand To The Sky
It Coursed Its Will Into Its Blade
And Made
Thrusting The Fountain From The Earth
Noelle
Take... The Pin... I Gave You
And... Stab The Earth
(If you had an edgy theory about making a fountain by stabbing someone to death, read on anyway, I won’t disappoint you)
The prophecy calls the EARTH 'she'. The Earth is a girl.
It says she will ‘expire’, and this will cause the Roaring - an odd term for mere rock, but appropriate for a person dying.
The sky will run black with terror
And the land will crack with fear.
Then, her heart pounding...
The EARTH will draw her final breath.
Only then, shining with hope...
Three HEROES appear at WORLDS' edge.
THE LIGHT AND DARK,
BOTH BURNING DIRE.
A COUNTDOWN TO
THE EARTH'S EXPIRE.
An anagram of EARTH is HEART.
Dess is what’s enabling the dark worlds.
Maybe she’s plugged into a machine that powers the Hometown or Dark Worlds simulation, if you want to go full Gaster on it. The ‘determination extraction machine’ in True Lab resembles a deer skull.

Dess is trapped in the code, and she is dreaming.
Controlled by the Queen's wire, it's sleepwalking through a nightmare!
WEREWIRE description
Whose dream is this?
Kris's dream?
Susie's dream?
Whose dream is this?
Knight's dream?
My dream?
Whose dream is this?
... you left it here.CH4 NPC
Every time the Knight makes a new cut in the earth, Dess gets closer to dying for good. The Knight is stabbing her to death. Whether it knows it, or not.
Consider: If the Knight actually wanted the Roaring to happen, they could have just made multiple fountains when we weren’t there to stop them. It has a different goal. It does not want to kill Dess, at least not yet.
This could be because it is a Holiday parent, it's actually a good person, or because Kris has bargained with it.
A fun thing about this theory is that it validates all other Knight identity theories.
If every Dark World is a haunted house with Dess as the ghost in the walls, then all of them can be influenced by her. She doesn’t need to be the one who actually made a fountain, for her to impart her will onto it.
A lost and lost and lost girl.
A crying and crying and crying boy.Rabbick text
King and Queen
The characterization Dess and Kris made for the Darkners when they played with the cards and laptop would make it into the Darkners themselves, even if the Knight had a different experience with the objects that became those Darkners.
Noelle’s Ferris Wheel
Dess’s love for her sister is so strong, she subconsciously created an attraction for the city she wanted Noelle to see.
1225 and Snow
These obviously have to do with Dess’s disappearance, so they would traumatise her enough to show up multiple times.
Trapped in the source code
The unseen person who pressed their hand against yours in CH4, and the person in the unused code, could both be elements of Dess's conciousness.
Ralsei
Ralsei’s similarities to Asriel may not have anything to do with Kris.
Dess loved Asriel and grew up with him, so her positive memories of him might have made it into Ralsei.
Gerson
One piece of worldbuilding about the earth is pointed out twice.
A dead monster’s dust is specifically put in the earth to connect them to ‘the divine’.
... When monsters die, their dust will be spread over what they loved.
An object that symbolizes their existence. That object will be buried...
And in such a way, their soul be able to rest, within that object, and the earth.
As per the ritual, a hammer is buried in the earth here.
To symbolize his existence.
To connect his spirit to the divine.
Gerson, as we know, was buried in the earth before being dug up.
This would make perfect sense with how well characterized Gerson is in chapter 4, if Dess knew him.
Cursed Sister
Susie mentions your sister is a party member with sucky stats in Dragon Blazers 2.
Having Dess become free enough to become a party member in the late game would satisfy this requirement.
It's like when your sister gets cursed in Dragon Blazers 2.
Even if her stats suck, she's still your party member.
So I don’t think Dess is the Knight. I think she is more important than that.
Maybe she’s the reason you’re here to begin with…
Don't forget
I'm with you in the dark
My Other Deltarune theories
Mettaton is the Knight
Tenna's getting possessed
Rosegrave, Or Who Made Weird Route?