Strictly speaking, the boy Manolin is not necessary to the story as far as the plot is concerned; the important events of the story occur at sea, with only the old man Santiago and the fish. However, Manolin serves an important role in humanizing both Santiago -- who is largely detached from others -- and the villiage, which is scornful of Santiago. Manolin's devotion to Santiago shows the hero-worship of the young to the old, even in their dotage, and his willingness to stand up for a friend against opposition, even when that opposition is his own parents.
"The hell with luck," the boy said. "I'll bring the luck with me."
"What will your family say?"
"I do not care. I caught two yesterday. But we will fish together now for I still have much to learn."
(Hemingway, The Old Man and the Sea, Google Books)
Manolin shows both concern for Santiago, and for himself; he knows that when the older generation dies without imparting its knowledge to the younger generation, something vital is lost from culture and humanity. Instead, he chooses to follow Santiago's example and become self-sufficient, not caring about what others say and instead helping those whose pride rejects pity. Manolin is the younger version of Santiago, the newer version, and he will accept Santiago's lessons and, later, teach them to the next generation.
Manolin is present only in the beginning and at the end of The Old Man and the Sea, but his presence is important because Manolin’s devotion to Santiago highlights Santiago’s value as a person and as a fisherman. Manolin demonstrates his love for Santiago openly. He makes sure that the old man has food, blankets, and can rest without being bothered. Despite Hemingway’s insistence that his characters were a real old man and a real boy, Manolin’s purity and singleness of purpose elevate him to the level of a symbolic character. Manolin’s actions are not tainted by the confusion, ambivalence, or willfulness that typify adolescence. Instead, he is a companion who feels nothing but love and devotion.
Hemingway does hint at the boy’s resentment for his father, whose wishes Manolin obeys by abandoning the old man after forty days without catching a fish. This fact helps to establish the boy as a real human being—a person with conflicted loyalties who faces difficult decisions. By the end of the book, however, the boy abandons his duty to his father, swearing that he will sail with the old man regardless of the consequences. He stands, in the novella’s final pages, as a symbol of uncompromised love and fidelity. As the old man’s apprentice, he also represents the life that will follow from death. His dedication to learning from the old man ensures that Santiago will live on.