Red Soul Trait Theory
I have a theory about the red soul. So you know how no one truly knows what the red soul is in Undertale/Deltarune? Well I think that it's nothing. Like literally nothing there is no special trait to it or more positively a blank slate full of potential. Here's my proof:
Evidence 1 The Ball Game
In the famous ball game from Undertale that we all know and love(or hate) if you get the red flag it reads out all the soul traits. We all know this but here's the thing determination is last and in a different text box meaning there is something special about it. That special thing being everyone has it and that's why it's red. In a theory I just came up with I'm calling "Rainbow Theory" I think that everyone has determination because red is the first color in the rainbow. It's a starting point the beginning of the colors and to get to the others you gotta start with red. Now I will admit this is one my weaker pieces of evidence but it gets better.
Evidence 2 Deltarune
In Deltarune from Berdly we learn that Kris was never really special. They were just some average C-Student with no real goals in their life and overshadowed by other people in their life. We also know that Kris didn't really have any friends besides Noelle so they weren't very sociable either. Now I know what you're gonna say, "The soul is a different entity from Kris so this doesn't prove anything" Yeah fair point, but counterpoint the soul is the player and the player is you and you could be anyone. For all I know you could be president of the USA or you could be some random guy eating chips and looking at Reddit. What I'm saying is that you could be anyone like the red soul could be anything which leads into my third and final piece of evidence.
Evidence 3 The red soul can change color
In both Undertale and Deltarune we can see changing color several times in the games such as when you fight Papyrus or Sans, your soul changes blue or in Deltarune when you fight Spamton, your soul changes yellow. This is even further backed up from Undertale Yellow which I know is a fangame but like every theory is gonna have some holes in them. In Undertale Yellow do you see Clover's soul change color? No it always remains yellow no matter what. That's because Clover's soul trait has already been decided it's Justice it's Yellow. While the red soul is nothing it hasn't decided what to be yet.
So that's it. That's why I think the red soul trait isn't one. It's a blank slate that is full of potential. Will this play any significant role other than changing soul colors? I don't know. We'll just have to see when Toby Fox finishes Deltarune in like 5000 years.