The marlin in The Old Man and the Sea is a worthy adversary for the old man. Like Santiago, he symbolizes grace, courage, devotion, nobility, and perseverance. Santiago says of the giant marlin that he battles:
Never have I seen a greater, or more beautiful, or a calmer or more noble thing than you, brother. Come on and kill me. I do not care who kills who.
In many ways, the marlin is a symbol of the old man himself. Both are calm, noble, courageous, and persevering, as well as devoted.
The devotion of the marlin is shown in a story the old man recalls of clubbing a female marlin to death while her mate was nearby. The male marlin stayed near her even after her death, earning the respect of the old man and the boy, who was with him on that fishing venture.
The marlin is also a symbol of Santiago's one last act of greatness. Santiago has to use all his strength and wit to catch this fish. When he gets into position, Santiago:
lifted the harpoon as high as he could and drove it down...and he leaned on it and drove it further and then pushed all his weight after it.
Like Santiago, the marlin will not give up the fight easily. He rears into the air, showing all his length and width and beauty. Santiago, emphasizing their symbolic relationship, calls him "my brother."
The marlin is also like Santiago in being at the mercy of the sharks of the world, who will stop at nothing to pick the creature clean. Santiago and the marlin are the noble creatures of the earth who maintain their grace and integrity in the face of evil.