Annie/Ukitake
Of course. A Hollow is a soul that has been overcome by emptiness. It attempts to fill the void by consuming other souls. Every Hollow assumes a new form when it loses its heart, and part of that form is a skull-shaped mask that hides the Hollow from who and what it was before. Its hunger is endless. To save the Hollow and the souls it had taken, a Soul Reaper must destroy the mask, and release the Hollow and its victims into the next life.