Manolin's relationship with Santiago uncovers aspects of Santiago's personality that would not otherwise be revealed.
At the beginning of The Old Man and the Sea, Manolin respects Santiago and wants to learn from him about fishing, but he respects the order from his father
that the old man was now definitely and finally salao, which is the worst form of unlucky, and the boy had gone at their orders in another boat which caught three good fish the first week.
As Santiago struggles with the sea, the great fish, and himself, he repeatedly thinks or says, "I wish I had the boy." Santiago understands that the boy would share the pride in this great catch and would appreciate the battle in which he was engaged; Santiago also acknowledges that the boy would be a real help in strength and endurance and in carrying out the tasks that were needed.
If the boy was here he would wet the coils of line, he thought. Yes. If the boy were here. If the boy were here.
When Santiago finally returns to the village, Manolin is the one who assumes the role of caretaker, bringing him food and beverage and giving him support and courage to look to the future, when Manolin will defy his parents and go fishing with Santiago.