It seems a bit strange to me that the fish plays such a huge part in the story, yet I had not really thought of it as a character. I am not truly sure whether it can be called a character, being a fish, but I am going to do my best to describe it as such.
For the fish to be a character, it would need to have both physical and emotional characteristics. I think the physical characteristics are the easier of the two. The fish is amazingly big and strong. He is eighteen feet long (Hemingway 58). He has a long and pointed sword at the end of his face and a really big tail as well. The old man thinks he is beautiful (Hemingway 45), and although I can not imagine a beautiful fish, I suppose I have not seen very many.
The fish may not have truly had an emotional character, but because the old man thinks of him as having one, he might as well. The fish is calm and noble in the old man's mind (Hemingway 45). The old man thinks of the fish as a worthy rival because of this. Because the fish pulls the old man for so long so steadily, he feels like the fish is "so fearles and so confident" (Hemingway 42). Because the old man thinks of the fish as having such a character, he feels very sad when the sharks eat him. The old man even apologizes to the half eaten fish for going out too far to catch it (Hemingway 55). He respects both the fish's physical qualities and the emotional qualities he thinks of it as having.
I am not sure if a fish can really have these traits, but this is a work of fiction. In stories like these, characters are exagerated to get a point across to the reader. In this case, the fish was given a character because the old man respected it enough to give it one. The author wanted to relate the old man's respect for the fish, and so he did.
Hemingway, Ernest. The Old Man and the Sea. New York: Scribner, 2003. Print.