The minor character I would have to focus on would be the Doctor, who plays a small but incredibly important part in indicating the way that the pearl will not increase the fortunes of Kino's family, as he hopes and desires, but actually decrease them massively. The doctor is initially shown to be a selfish individual who completely lacks any form of medical ethics. Even though Kino's son is in considerable danger of death, he makes it clear that he will only treat him if Kino has money to pay:
"Has he any money?" the doctor demanded. "No, they never have any money. I, I alone in the world am supposed to work for nothing--and I am tired of it. See if he has any money!"
Of course, his attitude changes completely when he hears of the pearl that Kino has discovered, and he goes and uses his knowlege and Kino's ignorance to deliberately mistreat Kino's son so he can swindle Kino out of some of his wealth. Consider how Kino responds to the doctor's words:
Kino felt the rage and hatred melting toward fear. He did not know, and perhaps the doctor did. and he could not take the chance of putting his certain ignorance against this man's possible knowledge. He was trapped as his people were always trapped...
The doctor therefore through his greed and duplicitous nature shows the way that the arrival of the pearl into Kino's life leads him to come to the attention of many such inscrupulous individuals, who are only interested in cheating him of the wealth he has gained.
This is a great question and I love how you are focusing on the minor characters. There is a lot of insights that we can glean by studying these minor characters. One minor character that is extremely important to the plot is the doctor. Although he is nameless, his actions, in a sense, drive the plot.
Kino goes to his to get medical attention for his son (Coyotito) and the doctor does nothing, because Kino cannot pay. Kino learns a hard lesson here and his innocence begins to become shattered.
The doctor represents the "real" world. He symbolizes greed, arrogance, colonialism in its worst, and the complete exploitation of people. This picture is amplified, because a doctor, if anyone, should know the value of life! His lack of compassion for Coyotito shows how inverted his values have become and how colonialism and acceptance of Europeans in higher society has poisoned him from decent values. Profits trump life.