Sorellas appear to be humanoid manifestations of a witch hat and are considered a rare sight among witches. This means that if their witch master dies they to will die. A sorella’s body and personality differ from the witch they are “born”, sometimes causing surprise with others for the extreme differences between the witch and her sorella (difference in personality for South and Peony and differences in body for East and Etwal). Because of these differences sorellas are treated as a different entity with a name of their one.
So far, the only witches revealed to have sorella's are among the great four; East and South.