Curate, connect, and discover
Something that I should also probably explain before diving into characters, is the way I think of souls.
As already explained in Undertale, the soul is the culmination of a being. I perceive it as if all of yourself is condensed in your soul. Memories, preferences, opinions. Taking one part out of that, the soul wouldn't change visibly, but physically something is missing, making the person holding that soul act, think and feel differently.
I also quite like the idea of the soul being reflected in my character's eye colours. That way, you kind of can look into their soul if you're looking into their eyes 😍.
I also really like the part with the soul traits. That means, you have one trait that you can categorise the way someone is.
The only problem I see with that is, that it's too bland. It would mean a brave character needs to ALWAYS act brave and a nice person needs to ALWAYS be nice to everyone. Because of that being unrealistic, but still wanting to include it, I like to think about it like your soul trait only shows at extreme moments. That way a character can still feel normal doing something that wouldn't exactly fit their soul trait.
If you take it a level further, it's also more likely for a character to drift into a constant behaviour that hurts their own soul. That means if a kind person is constantly being mean to everyone, it's going to affect them negatively. It could be physical but also mental. (Just came up with this last thought as I kept writing it down, but damn ... I see potential for some deep plot 🙂)