Patient Frisk and Courageous Dess (Undertale-Deltarune Theory Effort Post)
Well, calling this an Undertale theory is a bit wrong, as this will have prominence in Deltarune, but it's primarily about how Human Dess would work in the base Undertale universe (with implications that Dess is still a thing in undertale). For context on this theory, here's my blog on the overall Dess is a human theory and Shifters Inc's The Only Human Document (which theorises that Frisk is actually a resurrected Chara, with you having a similar role to Frisk as to Kris in Deltarune).
Now we know that Dess is likely in Undertale, though one thing to note is how their refered to. The alarm clock dialogue about Rudy's daughters is after the events of the true pacifist, and Sans dialogue about "the antlered girl and her big sis" is Sans reminiscing on the past. This, combined with the open sarcophaguses at the end of the true pacifist route, and the Only Human Document, establishes that Chara was able to revive herself as the amnesiac Frisk, which means it's very likely that Dess was able to be revived (likely alongside the other humans) and met up with her family. This would also mean, regardless of universe, that Noelle is gonna be truamatized by something happening to Dess (possibly even dying in both universes), regardless of the game.
Kris is the Patience SOUL
Now, for Kris, originally having a cyan (or patience) soul is a bit more straightforward. Kris's skin in the Dark World is a light blue in the dark world (notable for the most drastic change in colour palette amongst the lightners who've gone to the dark world). Kris is heavily associated with knives, even looking up online to figure out how to make knives with jello, and even carrying one around casually (a knife that looks similar to the toy knife in undertale). In the Omega Flowey fight, the cyan patience soul uses a whole bunch of knives (which are replaced by bandages, which is part of Kris's light world "equipment"). One of Kris's pranks involved hiding under noelle's bed to scare her, which would naturally require patience. Its also noticable that inverting the Red SOUL's color makes it into cyan. Kris is also able to equip Ribbons, and ribbons are your first piece of armor you can get in deltarune (its also notable that in the fun gang, only ralsai, whose origins likely has some connection to kris, is able to also equip ribbons). The Toy Knife and the Faded Ribbon are also found in the ruins, which is where Toriel hangs out, and toriel currently acts as Kris's
Dess is the SOUL of Bravery
This is a bit more of an out-there claim. We know they have the Manly Bandana and the Tough Glove (described as pink in the flavour text, and its steam badge art depicts it as a dull brownish yellow with an orange outline), and in the Omega Flowy Fight their represented by a bunch of white gloves with black spots on them (the inverse, interestingly enough, of the knight's black hands with white spots on them). You can also get both the Manly Bandana and the Tough Glove in Snowdin (which I don't need to tell you are probably where the holidays are).
There's also the matter of her interest in military things. Not only does she have military rations and walkie-talkies, but there's also the camo-patterned shirt in her closet. Maybe the bandana is meant to be a military bandana, or at least evoke one, but that's speculation.
There are two very interesting instances of the color orange in association with Dess. One, a more minor one, is Seam who is mostly orange with purple fur and their shop background is also shades of orange. Seam appears on Asriels' desktop (who we know is close to Dess in Deltarune, possibly even romantically involved), and Dess has frayed string and buttons in a box in her room and helped Noelle with making the angel plush doll.
The next is more glaring, and that's her guitar. In a room of cool purples, blues, and desaturated reds, this bright orange guitar stands out immensely. Especially since the code to the bunker (which we can safely assume is 1225, or Christmas Day) is in the guitar. I mean just look at the image below. A more minor note, but with the three humans we know about its interesting that Dess is the odd one out (being explicitly female). Yet the unused dialouge of chapter 3 and 4's japanese translation used gender neutral pronouns as opposed to chapter 1 and 2's feminine pronouns. Not sure what this means in relation to dess being human (maybe she started out as she/her before being trapped in the depths or whatever made her go they/them, or she started out as they/them and yo-yo'd her way back to it).
Okay, so Dess is the Bravery Soul and Kris the soul of Patience. What does this all mean for Deltarune? Well, first I do have some observations about the attacks associated with the Orange and Cyan Souls. That being, while cyan attacks are introduced in Snowdin (a possible further connection between the orange and cyan souls's relationship), when orange attacks are introduced, they tend to be used in conjunction with cyan attacks. Mostly via the various hotland lasers and Asgore (who in Deltarune has some interest in whatever fate befell Dess in this game).
Indeed, the two attack types cover each other well, with cyan attacks only damaging you if you move and orange only damaging yourself if you don't move. This synergy between the attack types reminds me of how Kris and the Knight are working together, with a sprite of a shadow monster in board 2 (implied to be representative of Dess) mirroring Hero Sword's movement. Kris and the Knight even share similar poses, and I would certainly associate bravery as a virute a knight would have.
There's also the matter of the Knight's resemblance to a pair of titans in the back of a slide from Ralsai's monologue about the Roaring (also note all images about the prophecy slideshow from chapters 1 and 2 are in shades of orange), possibly alluding to Kris and the Knight working together (and whiel this is speculative, possibly hinting towards frisk becoming a similar entity to the roaring knght down the road). Hell, if the sword route minigames are to be believed, Kris either presented himself as the roaring knight or at least acted as a middleman for the roaring knight's deal with Tenna.
It also makes me wonder if whatever affliction that's affecting Dess and turning her into a knight is also affecting Kris. Kris's occasional slow zombie-like motions, and his skin colour, which is reminiscent of a corpse or someone being asphyxiated, definitely scream that he's in some sort of undead state (or something akin to it). Compare that to the skeletal, amalgamate-like Roaring Knight, who's just straight up ripping their ribcage open as part of their final attack. This is admittedly mostly speculation, going off how the two of them were similarly revived from the dead at the end of Undertale (with their fate in Deltarune being sort of a twisted version of it).
As for how this relates to the vessel, my current bet is that the second voice (either Kris, Dess, or FRIEND) hijacked the goner-maker sequence to get Kris the Red SOUL for whatever plan they are cooking up. Said Goner Vessal is likely intended as Deltarune's Chara/Frisk analogue before the second voice interrupted the sequence.
