Does Jacob really need a physical entity to exist? My thought are "No." My guess is the island is Jacob. Jacob can be anything person Jacob wants to be to communicate with the living world on the island.

Jacob is the Island;the whispers, and all of the various apparitions. He is also connected to the process of the transmigrations of the souls of the dead-that is another of the Islands properties-why else would a ghosthunter/spirit medium like Miles have been sent?