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The doctor is a small but important figure in the book. When Coyotito is bitten by a scorpion, Juana and Kino try to get a doctor. As Kino is about to go, the people say that the doctor will not come. In fact everyone says he will not come. This point is actually emphatic. Here is what the text says:

"He would not come," the people in the yard said. "He would not come," the people in the door said, and the thought got into Kino. "The doctor would not come," Kino said to Juana.

The reason why the doctor will not come, and the people are correct, is because the doctor will not treat people who do not have money. He does not really care about people. This is not why he became a doctor. What motivates him is only money. 

In the end, the doctor does make a visit, but he does so only because he comes to know that Kino found a pearl of great price. The doctor, then, represents arrogance and a spirit of colonialism. This comes out clearly in the text:

The doctor had once for a short time been a part of the great world and his whole subsequent life was memory and longing for France. "That,"he said, "was civilized living"- by which he meant that on a small income he had been able to keep a mistress and eat in restaurants.

The doctor makes Kino and others resent people with power. At the same time, Kino wants what the doctors wants as well - a good life, defined by wealth.  Herein lies the irony.