SPOILERS FOR ALL OF DELTARUNE
Pre chapters 3 and 4 a theory I swear I saw floating around was that all of the secret bosses would each use a different soul mode (jevil red, spamton yellow etc) but then the chapter 3 secret bosses threw it off before returning to the pattern in chapter 4
You'd think that since there's only 7 chapters and 7 souls, the soul mode thing wouldn't work, but if you're really willing to stretch it, the feather sections in chapter 5 kind of have the same properties as the blue soul and then chapter 5 secret boss has it's own soul mode
The final boss of chapter 5 ALSO uses a unique soul mode that's a total of THREE soul modes in one chapter. Now, the only one that's left now is the cyan or aqua soul. Who is that exact color? Kris.
Now side note/theory, I believe the soul we play as was not Kris's original soul and that they had a different soul prior to the events of the game. Noelle comments on how different they act throughout the entire game, especially in the weird route, meaning that they were completely different before, but in the opening cutscenes of chapter 5 we see them in very poor condition after running around with no soul at all, meaning that they A, need a soul to live, and that Deltarune souls operate using the same logic as undertale souls (furthermore in chapter 1 when Susie fights lancer we play as the red soul and not Susie's soul) and B, Kris doesn't have their own soul when they get rid of the red soul. You could argue that they pop their soul out all of the time, but most of them are for a few hours max, while at the beginning of chapter 5 they've been missing their soul for the whole night all the way to morning.
I believe that Kris's soul was cyan and (that's one of the reasons they're blue and) they must have somehow replaced it with the red soul which we, the player, control. Probably as part of the conspiracy they're in on, because it requires doing something that only a human can do, and Kris was incapable of doing it with their original soul. (Cyan being patience red being determination, maybe the task required determination, like queen says you need to focus your determination into the blade to make a fountain, but we see Kris make them without us so that might not be it)
And all of the leads to my theory that the VESSEL (that's right we're bringing it back) is the KNIGHT, and the reason it has traits from both Kris and Dead is because it has KRIS'S SOUL and it's imprinting off of their experiences and memories!
That one's a joke actually but I do believe that the cyan soul with return somehow and we will have to either fight as it or against it.
And Aqua's relationship with Kris kind of fuels this theory for me.
