First of All, I have to tell you guys why I make more souls for the history:

I want to make this history more... original, just make the same history with te same things and with some changed details would be boring, so I want to improve a little bit.

As you may know, Game!Sans has one of the human souls (This was said in the info of this AU), and for this I have to put one more to Asgore have 6 Humans souls, if I dont add one, Sans would have to sacrifice himself to open the barrier, and we all know he missed out on getting out of the underground, so he would not do that.

Unity Soul:

This soul is more them just a “substitute” for Perseverance in the story, really.

They will have references and quotes about them connection to the monsters while they was alive, not like a full story but I think you guys know what I’m talking about.

Hate Soul:

If you guys read the info of the AU, you can have a notion what is this soul, but I dont want to give spoilers for you, so I go stay quiet.

No Trait/Magic Soul:

Basically they are a large part of the souls of humans, only few possess magic, so the presence of this kind of soul is also necessary for history.

Monster With Human Soul:

It’s not a “super new soul” but is necessary  too, Original Asriel have one (But the background soul would be red), and Game!Sans have the extra soul with perseverance.

If any other monster absorbs a human soul, they will have a similar soul, except that the background color would change. The gray soul without magic can also be absorbed, but the differential is that the monster would be less strong and without magic of a trait, just determination.

Why Red is not Determination?

I already wait for this kind of question, so let’s talk about this, why not Determination? Because every human have determination, Frisk’s soul would be a simple “more powerful thing that every human have a little”, and in the game, just some things show the real trait of Frisk/Chara/Player, and you guys can read about this here.

You can also refer to the red soul as “individuality” or “nonconformity”.

Thanks for @nochocolate for the theory about red Soul