Kino is the protagonist of the novel, although his wife Juana is also a central figure. These two characters are not entirely on par with one another in their importance to the text and so do not quite share the role of protagonist.
Kino is the figure whose decisions drive the action of the novel. His anger and his ambitious dreams serve to create the situations wherein the family is put in peril. Also, importantly, it is Kino's awakened sense of agency after finding the pearl that presents a large portion of the complex theme at the heart of the novel.
While Kino was happy as a poor fisherman, he had no agency -- no sense that he had a power in himself to change his circumstances. After discovering the pearl of great worth, Kino begins to think of himself differently, discovering also a sense of individual power and personal agency.
His new-found potency of spirit is quickly tested and quickly shown to be as much aspiration as reality. Although he is wealthy, Kino remains set upon by those in power and by those who associate themselves with power. As an Indian from the fishing village, Kino's agency proves to be a challenge to the power structure of the local culture and ultimately endangers Kino and his family.
Kino's complacency at the outset of the novel is replaced by a sense of new capabilities, but in finding his power Kino also finds his limits. Perhaps his changes lead him to embrace generic and materialistic desires. In hoping that Coyotito can go to school, Kino is hoping his child can escape the circumstances of oppression and/or exploitation that characterize his life with Juana in the fishing village.
“For it is said that humans are never satisfied, that you give them one thing and they want something more. And this is said in disparagement, whereas it is one of the greatest talents the species has and one that has made it superior to animals that are satisfied with what they have.”
So, if Kino's burgeoning sense of self causes him pain, loss and unhappiness, it is not only because he became arrogant or drunk on wealth. His pearl makes him feel that possibilities are open to him and his family that were once closed.
Again, in considering the question of who is the protagonist in the novel, we can point to Kino as the character that makes the important decisions and as the character that experiences the greatest growth and change in the narrative.