There's a picture of some souls up in the permissions post info. It's the "Aren't they cute" statement. Plus there's Kid's soul that's in the post icon. Largely, imagine an orb with a wiggly tail and a face and you've got the general idea of what a person's soul looks like.

Also, points for trying to make the acceptance notice centered. Kid appreciates it.

I'm thinking maybe the soul's appearance could look "shiny" and like it itself might be a crystal with little blue wings coming out of it? And appropriate that his soul has wings since in Soul Eater world, that makes his soul a "grigori" soul which is basically an angelic soul.