[s]arrgh why did you eat my post internet rargh[/s]

Hollow souls are souls that have literally no alignment until they come in contact with a purely black or white soul. They can then take on all the traits of that soul.
There's also Dual souls which are one body two minds, like for example Jekyll and Hyde.
Twin-souls are conjoined twins who share a part of their soul with each other.

As for abilities these are more specialized to each character and I also want to keep tabs on what powers special souls and regular souls get. Naturally under soul alignment I would need a description for special souls. What their drawbacks and strengths are, for example.