The old man is a character isolated from people – and in fact from the world of humans entirely – in his time on the sea. This isolation defines who he is, and emphasizes the unique nature of his character. Isolation becomes both a weakness (he suffers from loneliness), but also a necessary element to his battle with the fish. Although the old man accepts and admits help from others, it is alone that he does battle with the marlin, that he must prove himself and his capabilities.
Isolation is a necessary element of the old man’s struggle with the marlin. Although he wishes for the boy to be present, he must do battle alone to truly prove himself.