The Marlin and the sharks are representative of Nature, which in Hemingway's parable of The Old Man and the Sea are archetypes for the moral of the existential struggle of Santiago as both a man and a fisherman.
Ernest Hemingway's novella The Old Man and the Seahas only two primary human characters--Santiago (the old man) and Manolin (the boy). When Santiago is at sea, he catches the marlin and he calls the great fish his "brother." The marlin is alive and makes decisions and reacts to what Santiago does, so he does qualify as a character.
The sharks are certainly antagonists to Santiago, so a case could be made for their being characters, as well. They interact with both Santiago and the marlin and are a primary source of conflict in this novella.
Santiago's hands are a little different. If one chooses to see them as allies or enemies (depending on if one or both are working), they could certainly be considered characters. If one sees them merely as extensions of Santiago, they could probably not be called characters. A very interesting idea to consider!
Perhaps we can consider the bird to whom he was talking also as a character, since there is some kind of conversation between them.