Aminadab is a total oddball character. There isn't a whole lot of text devoted to him in "The Birthmark," but what is there speaks volumes. Hawthorne describes Aminadab as "a man of low stature, but bulky frame, with shaggy hair hanging about his visage, which was grimed with the vapors of the furnace" (26). He is actually a little creepy, if not vaguely sinister. We learn that he isn't capable of understanding the science behind Aylmer's work, but that he can execute all the physical details easily. And then, of course, we have the very direct line:
With his vast strength, his shaggy hair, his smoky aspect, and the indescribable earthiness that incrusted him, [Aminadab] seemed to represent man's physical nature; while Aylmer's slender figure, and pale, intellectual face, were no less apt a type of the spiritual element. (26)
That's our first tip-off that we're dealing with a symbol here. Aminadab symbolizes man's earthly, physical half; Aylmer symbolizes man's lofty, spiritual half. Our second tip-off is his name. "Aminadab" is the name of a high priest in the Bible, so it's not a stretch to think that Hawthorne's character is also meant to symbolize men of religion, in contrast to Aylmer, who symbolizes men of science. The first thing we notice is that the man of religion is, in this story, subordinate to the man of science (Aminadab is Aylmer's servant). This isn't a value judgment on Hawthorne's part; rather an observation of the times in which he is living. See both "In a Nutshell" and "Setting," where we discuss the ideological environment of "The Birthmark" and of Hawthorne's own time. You'll see that science was on the rise as compared to religion.
At first, it's hard to reconcile the fact that Hawthorne equates science with loftiness/spirituality (in Aylmer's character) and religion with earthiness/physicality (in Aminadab's character). We would expect the opposite: the spiritual man is the religious man, while the earth-bound man is science-driven. But this is not the case. We can understand these associations by recognizing that Hawthorne is interested in a particular kind of scientific inquiry – the kind that, perhaps in Hawthorne's opinion blasphemously, is built upon man's spiritual aspirations to conquer nature and become like God. Aylmer isn't exactly experimenting with pea plants here; he's trying to remove the symbol of man's inherent flawed spirit. That's why science = spiritual.
Notice that Aminadab, unlike Aylmer, has no such aspirations. "If she were my wife," Aminadab says, in his only line of dialogue, "I'd never part with that birthmark" (28). This is the first time we can sense that Aminadab knows something Aylmer doesn't. He may not be able to follow Aylmer's science, but when it comes to wisdom, he just might have the edge. To the man of science, becoming God is a lofty and noble goal. But to Aminadab, it is blasphemous, impossible, and certainly unwise to try.
The last thing to talk about here is that laugh at the end of the text. When it becomes apparent that the birthmark is fading from Georgiana's cheek, Aylmer hears "a gross, hoarse chuckle, which he had long known as his servant Aminadab's expression of delight" (85). We hear nothing more from the Aminadab until Georgiana dies; then "a hoarse, chuckling laugh was heard again!" (91). We see that religion has triumphed over science in that Hawthorne's tale has proved the foolishness of trying to become God; religion, despite having been made into a servant by science, has the last laugh – literally.
But the reader can't safely take Aminadab's side here; in one sense, we're right back to where we started with that slightly sinister tinge to his character. He laughs at Georgiana's death. In this way, Hawthorne complicates his story considerably. He's certainly questioning the wisdom of letting scientific inquiry drive our entire ideology; but he's certainly not so positive about religion, either.