Quick reminder, Shmoopers: The Pearl is a parable—the intention of this story is to illustrate a moral lesson rather than tell a story complete with nuanced, complex characters.
Why are we reminding you of this? Because The Doctor is about the least nuanced character in this entire novel. He’s a caricature of everything that’s wrong with colonial oppression. From the "little hammocks" of his eyes to his dressing gown of red watered Parisian silk, the doctor eats, sleeps, and breathes greed:
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. He poured his second cup of chocolate and crumbled a sweet biscuit in his fingers. The servant from the gate came to the open door and stood waiting to be noticed. (1.37)
With no respect for human life, he uses his profession only for money... even if that means manipulation. He tends to an old woman (though her only ailment is age) yet he refuses to treat Coyotito’s potentially fatal scorpion bite. His desire for wealth has completely skewed his sense of values.
But the Doc hits a new low when he comes to visit the newly-rich Kino under the pretense of treating his son. For a doctor to poison a patient intentionally is absurdly unethical—and more evidence that this particular doctor is a symbolic figure (who basically has "Colonial Villain" tattooed on his forehead).
As if insane greed and a complete lack of ethics weren't enough, the Doctor is also incredibly racist. Before he refuses Kino on the grounds of his poverty, he is disgusted by the man’s race and remarks that he doesn’t treat Indians as he isn’t a veterinarian. Yup, he's saying that Kino is an animal. This guy is just so horrible.
But you know what? Colonialism is, um, so horrible. When you read through the atrocities committed in the name of empire-building—in Mexico or elsewhere—you start realizing that The Doctor is a compilation of a lot of the evil stuff that went down in colonial outposts. The racism. The greed. The total lack of morality.
But in terms of The Pearl, we need the Doctor around for a few reasons. First of all, his (lack of) actions prompt Kino to go pearl diving and hit the pearl jackpot. Second of all, his statements that compare natives to animals tragically foreshadow the fact that Kino devolves into animalistic behavior in his attempt to save his family—it's not Kino's fault that he has to turn to animalism; it's the fault of the society that surrounds and suffocates him.