It is clear that throughout the novella Kino changes greatly thanks to the introduction of the pearl into his life. He starts off as your average hard-working Mexican fisherman, desperately trying to provide for his wife and son and battling against poverty and discrimination to succeed in this goal.
However, with the advent of the pearl into their lives, which should have been a blessing, it is clear that evil only comes. The pearl's malign influence distorts Kino's perceptions of reality and causes him to become absolutely fixated on getting the right price for the pearl so that he can allow his son to break out of the poverty trap that has dominated these lives. Pursuing this goal, however, causes him to change. Note what he says to his wife when she insists that the pearl is evil:
And as she spoke the light came back in Kino's eyes so that they glowed fiercely and his muscles hardened and his will hardened.
His face becomes "crafty" when he thinks of what he must do to sell the pearl. In the next chapter, when Juana tries to dispose of the pearl, he hisses at her "like a snake" with bared teeth, hitting her. However, perhaps most chillingly, in spite of all of these transformations, is what Kino says at the end of chapter five:
"This pearl has become my soul," said Kino. "If I give it up I shall lose my soul."
We see here the way that Kino's life has become completely dominated and obsessed with the pearl. He links it inextricably to his soul, indicating the malign influence that the pearl is having on him. Of course, with such an influence, only a tragedy is able to break it, and the death of his son gives Kino the strength he needs to return the pearl to the sea from whence it emerged.