Is Manolin necessary to the book? (Describe the role of Manolin in the novel.)
The Old Man and the Sea is, essentially, the story of a single character. Other than the old man, only one human being (Manolin) receives any kind of prolonged attention.
Manolin is essental, I think, to The Old Man and the Sea. Whenever you have a story about an old man, you must have a young boy as counterbalance. As such, he is a foil for Santiago in that the boy is a younger version of the old man. Like Santiago, he is loyal, patient, and selfless--all the qualities of an aspiring fisherman. More, he is the only one on Santiago's side. Whereas the young, arrogant fishermen make sport of the vocation--fishing for money--Manolin is learning to fish out of love, honor, and a duty to nature.
As the saying goes, "If you give a man a fish, he will eat for a day; if you teach a man to fish, he will eat for a lifetime." Manolin's other role is to be a disciple to Santiago, the Christ-figure. In part I of the novella, we see Santiago continuing to be a mentor to the boy even though the old man has gone 84 days without catching a fish. His resolve to continue fishing despite his bad luck is a valuable lesson learned for the boy. In the end, after Santiago returns with the marlin carcass, we know that boy will be the one--like Christ's disciples--to spread the gospel about Santiago's epic catch. We also know that Manolin will grow up to be a fisherman in the mold of the old man.
On a literal level, Manolin is there to be a receiver for and believer in Santiago. In part I, Santiago must have someone to talk to. He can't talk to his daughter or the other fisherman; they don't listen or care. But, the boy looks up the old man: he still believes in the mythos of the young Santiago, the one of arm wrestling and fishing greatness. So, the boy is there for balance: Hemingway cannot have the old man talking to himself for the entire novel. Santiago talks to the boy in the first half and to himself in the second. Still, Santiago is speaking to the boy the whole time, if not literally, then spiritually. When he is alone on the boat, Santiago wishes the boy were there, and he even speaks out loud to him as if he were.