To explore Kino, the main character of the novella, would be the logical choice. Steinbeck portrays his transformation in a very clear way. At first, Kino is a very simple man. He loves his family, his people's traditions, and he is content with his life. He lives in a small hut and he dives for pearls for a living. However, all of this changes when his son, Coyotito gets sick and he finds a pearl of great price.
At this point, Steinbeck shows that greed and ambition get the best of him. All of this starts very innocently, as he can picture in the pearl, a better life for his son, such as an education. Then, he starts to dream of wealth. He also gets a sense of false confidence in his new treasure, which leads to the ambition of others as well. In other words, they want to steal the pearl.
The reader gets the sense by this point, Kino is a different person. His nature has been tarnished. The innocent Kino now becomes violent. He is now a man who will do anything to reach his objectives. At one point, he even strikes his wife, who wants to get rid of the pearl. This violence escalates as he kills a man, who wants the pearl for himself. And towards the end of the story he kills another. We can say that innocence is completely lost. Kino is beastly.
Finally, at the climax of the story, Coyotito is killed. This leads Kino to reassess everything. He comes back to town and throws the pearl back into the sea. There is a sense of disillusionment. He is now a completely different person and one gets the sense that nothing will be the same.