Note I just made this up but i doubt they will be explored because most of them are minor members of the organization.

Demyx - Was too lazy to run from a heartless because he would rather take a nap, so he lost his heart.
Luxord - Lost his heart as part of a gamble. Porfessional poker player by night. Slot machine addict by day.
Marluxia - Former Florist/assassin for hire. His world gets devoured by heartless. Possibly a crossdresser.
Larxene - She possibly worked as an electrician or given her personality she administered electroshock therapy. Lost her world.