Coyotito is the infant son of KIno and Juana. He was stung by a scorpion and nearly died, but his mother relieved the sting. After his parents found the "Pearl of the World", the doctor poisoned Coyotito to convince his parents that they needed to pay him for treatment.
The doctor is a man of a different race than Kino and his family. He is greedy and refuses to help people who sometimes can't afford it. He wants the Pearl when KIno finds it. He goes as far as to poison Coyotito in order to convince Kino that he needs to pay for treatment.
The priest is a man who, despite him trying to act as though he cares for Kino and his family, only cares about money for the church and converting the natives. When Kino finds the pearl, the priest immediately imagines how the money could help the church, then acts as though he is on Kino's side.