Parkhill is a guy who would go all the way to Mars just to set up a hot dog stand. When a Martian comes to talk to him, he threatens to give him "the Disease" (22) and then "go on and hide in the hills, that's where you belong" (24). He can't appreciate Martian art, so he crushes their glass to make a pretty walkway (8). He gloats that Wilder is off "freezing to death" on Jupiter while he's about to make "thousands" on his hot dog stand.
Want more proof? Check out what he says to the visiting Martian:
I mean you harm… I don't like strangers. I don't like Martians. I never seen one before. It ain't natural. All these years you guys hide, and all of a sudden you pick on me. Leave me alone. (26)
So, is there anything good about Sam Parkhill?
Not really. He's a perfect minor character for showing off what's wrong with the colonization of Mars. We have no idea how he ever convinced Elma to marry him, but considering that he threatens to kill her, we're not sure how long that marriage is going to last (100-1).
Elma is Sam's wife, and, like a lot of spouses, she knows the truth about him. Sam will say that he's too quick for the Martians; Elma will say that they let him get away (77-8). Sam gloats about the 100,000 new settlers who are supposed to be on their way; Elma basically says that she'll believe it when she sees it.
We don't ever get to really know Elma as a character—what she wants, why she married Sam, how she feels about Mars. We just know that she married Sam and goes along with his ridiculous plans (including his plan to bury a Martian that he shot). When there's a nuclear war on Earth, she doesn't sympathize with the victims, and she just uses it as an opportunity to be cruel to Sam.
So Elma may tell the truth, but that doesn't necessarily mean she's a sympathetic character. She does get to deliver the final punchline to the story: Sam won't get rich from his new lands and his hot dog stand because "This looks like it's going to be an off season" (189).
Maybe they deserve each other, after all.