From MangaHelpers:

There are men and women that have the ability to control their shadows, molding them into shapes according to their will. These "Shadow Users" gain this special ability through some traumatic event from their past, creating darkness and sorrow in their soul. Cole is a Shadow User, but his shadow, named Shiva, was once human. Now, he is on a search to find the individual responsible for turning Shiva into a shadow and return Shiva to his body. [DrCoke]