Could Asriel be the Knight?
Going to preface this by saying I don't really listen to many theories, if any. I can't catch up with them, so I don't know if any of this has already been talked about at length or not.
However, I see a lot of people linking the roaring knight to Dess, Gaster, Papyrus (?), Rudy, Carol... Etc. Though I'd definitely considered Carol due to her odd behavior, the way she talks to Kris, the horns! etc... I'm starting to make a bit of a crack theory. Maybe Dess and Carol are a red herring while we're forgetting the part where "Asriel is coming back from college" is told to us from the very beginning of the story.
He feels like such a prominent part of the puzzle, and yet I rarely see people tie him to this.Aren't there also parallels to Asriel? Even design wise, it's not too far off from the God of Hyperdeath. The hollow chest, the holes in the hands, the shoulder pads (?) The motif of hopes and dreams, but inverted?
From the very start of the game, we see a very intentional presentation of Asriel without even knowing his character:
He's a medalist, multiple award winner, distinguished graduate, talented artist! We then learn he's a bit of a nerd, he loved Dess, he was a great part of his community, with a great reputation...
But all of these are external observations and anecdotes of Asriel, never his internal perspective. Even Kris' perspective on Asriel is idealized, heroic, even.
We never see who he is objectively. Looking at the Dreemurr family as a whole, they'd collectively been dealing with a difficult family dynamic... For years. And if Undertale taught us anything, it's that Asriel as notorious for shouldering everyone's burdens with a smile. (And as a churchgoer, wouldn't he also be just as aware about the prophecy? About anyone being the hero?)
What if Asriel lied?
What if he had never gone to college, and instead hid from everything in the shelter? What if he'd also been suffering from the family slowly coming apart, the times they'd enjoyed becoming distant memories, Dess disappearing, maybe without him knowing why... (or maybe, they'd escaped together?) What if he was just as lost as Kris is now, and Toriel/Asgore didn't notice?
The Knight's sword is made of shadow crystal, which Seam (Asriel's worn plushie, which Kris returns to the house, by the way,) reveals are failed dreams. (Side tangent: It's also really interesting how you can still interact with Seam despite him not being in the supply closet. Is castle town the town itself? Is that what "ROOTS" are, given the literal display of a home surrounded by trees? Ralsei's ability to go into any dark world certainly supports this. Did it start in their bedroom?)
If you put Asriel in the role of the knight, he could have a lot of motives for failed dreams- especially to use as his weapon, his tool to move forward. Dess, Asgore's distance and obsession with the dark world, the divorce, the Holidays growing more detached, things falling apart around the family, Kris acting incredibly different after getting possessed, and maybe, just maybe, lying to the others about going to college, and hiding in the shelter instead. It's especially possible now that we learn that Asgore was especially involved in studying it, meaning Kris wasn't the only Dreemurr invested in this.
What if he just wanted... To run away, and create a dream where life goes on, their family and friends are all together, Toriel recognizes that Asgore was telling the truth, and he can find Dess in the deepest, roaring dark? It's also possible Kris played with the card darkners, Seam, and everyone else... With Asriel. As an escape from the world that was crumbling around them. (Makes me think about Mancountry.)
If the knight is Asriel, it makes even more sense why Kris is doing the absolute best to keep anyone, us included, from finding him. Deleting numbers, avoiding opening the door knowing "a large person could fit inside" the chapel, avoiding the guitar...
"Susie... must not get... guitar..." Because if they found Asriel here after lying about being gone... Everything would fall apart with questions, right?
Asriel likely knew about the shelter code from Dess.
And, if you look at the ominous call Kris had received back in Ch 4, you get lines like:
.. Kris... dark world... no soul... can't...
"...need... soul..."
Without... soul... Kris... will..."
Doesn't it sound like they're talking to someone else besides Kris?
And... Don't they almost sound concerned for Kris...?
I think that Asriel is just as likely to want to escape his role, like Kris, Noelle, Susie, and everyone else in this story has tried to in their own ways, using the Dark Worlds.
That, or the vessel we'd made in Ch. 1 went amok and Kris wants to swap us out with whatever it can give in order to free itself from control. I'm sticking with Asriel for now.
EDIT: Other things of note:
Asriel has proven to have a lot more going on than we'd been able to glean from flavor text alone, and Ch. 3 hints at this the most- we think he likes "Green Lizard" and he prefers "Sword Magician" in time. We think of him one way, and the game subverts our expectations of him, already telling us not to expect to know the full story.
Kris seems most defiant when it comes to protecting others, but otherwise seems quite defeated when it comes to Kris' quality of life. Whenever the Knight or clues about the knight's true identity come into play, Kris tries especially hard to forget, distract us, or forgo interacting with the world entirely. This suggests more rapport than just "Carol is telling Kris to find her daughter."
The Roaring Knight is extremely versed in moving around the dark world- and has Dark/Star attacks that can only be defended against in part with the shadow mantle. Dark/Star certainly seems to fit the bill of the God of Hyperdeath's space-themed battle.
The Ch3 fight.
The Roaring Knight rejects their mercy, but is exceptionally merciful to Kris, only attacking Kris to make the battle convincing despite pulling punches when dealing max (999? seem familiar?) damage that doesn't heal with turns to keep them from interfering and getting their way via defeat/sparing. The battle itself is overly favorable between Kris and the Roaring Knight- Roaring does little damage and Kris willingly stays down.
Kris barely does damage even with good weaponry. Despite being so protective of family and friends, Kris makes sure to leave the door open and watches idly while the knight carries Toriel away.
Kris offers a cue only the Knight can pick up, coughing before the end of the battle. Wouldn't the knight have to know this is one of the few ways Kris can openly act?
In Ch 4, we know the Knight had opened a fountain when the church was empty due to the rainfall. But when we go into the final battle... We see that the Knight had been moving around, stopping us from fighting directly... And looking for someone. It could be a Holiday, sure... But it could also be Asriel, because again, Kris knows who could fit behind the door. Kris starts to tell us things like, "Then again, you never liked looking closely."
EDIT 2: I misread, Seam is not in the Dreemurr home! It appears he is still in the supply closet. I still believe that there is a strong argument for castle town, mancountry, and similar dark worlds having been started at home or school.
Seam feels like such a big part of the puzzle, too; he actively understood and awaited the coming end of the prophecy (potentially even the world) since Ch1. He proves time and time again to be an incredibly powerful magician- more than Jevil or any of the mizzles, making him another example of well-loved items being stronger: Flowery, Seam himself, the family TV, etc. (Could Ralsei be a well-loved item by Kris, hence his odd infatuation with Kris? Or... could Ralsei be something connected to US?)
That's all for now! Thank you for reading!