D's constant companion is his talking left hand, sometimes reffered to as the Countenanced Carbuncle. It is revealed that this is some sort of symbiotic entity which has taken up residence in D's hand, although why he has done so is not clear. The hand (or "Lefty" as fans have affectionately termed him) appears to have supernatural powers independent from his host, and occasionally uses them to help D. More often than not, however, Lefty generally prefers sitting back and letting D do the work, usually throwing in a sarcastic comment or two at D's expense.

(Source: Vampire Hunter D Wikia)