Expecto Patronum! The French apothecary Pierre Pomet writes that the very bones of deer could "revive the Spirits expel Melancholy, and help the Palpitation of the Heart." All three Potters—Harry, father James, and mother Lily have the fleet-footed deer patronus in common (James is also a stag, while Lily is a doe). No wonder the appearance of Harry's agile animal spirit and guardian steadied him during his face-offs with the most evil wizard of all time.