We admit that it's weird to call "Americans/Humans" a character, but it's hard to shake the feeling that the really important figure in this book is humanity itself. And most of the examples of humanity we see just happen to be from the United States.
So pretend for a moment that there's a single generic character in these stories called "Americans/Humans."That character would be so inconsistent you'd want to medicate him for Dissociative Identity Disorder. (You know: multiple personalities.)
There are as many motivations as there are people having them. For example, in "—And The Moon Be Still As Bright," Parkhill is a moron who only cares about selling hot dogs and making money, but Spender cares about the history of Mars. Benjamin Driscoll from "The Green Morning" cares about something more than what he can earn or take from Mars. (After all, he isn't making money by planting trees—he's just doing it to make the world a better place.)
But there is one thing that all the humans want: to get away.
Some of them want to get away from sad memories, like the LaFarges who come to "enjoy [their] old age in peace, not to think of Tom" ("The Martian," 11). Some come—or want to come—to "get away from wars and censorship and statism and conscription and government control of this and that" (The Taxpayer," 1). Some come "to get away from being instructed and ruled and pushed about" (4-5).
Some, like the African-Americans of "Way in the Middle of the Air," come to escape racism and other social injustice. Some, like the women of "The Wilderness," come to marry, escaping lonely lives on Earth. And some come to escape the "mechanical wilderness" of an Earth whose technology got ahead of its sense ("The Million-Year Picnic," 123).
Are you getting a picture of humanity, here? It's restless. It wants more, it wants better, it wants new. And, sure, we can see this is a good thing. The quest for something better and newer is what got us into space in the first place, not to mention iPhones, self-driving cars, and vacuuming robots.
But (obviously) there's a downside. Because it turns out that there are already people living on Mars—but not for long, not after humans get there. And, no matter how far they go, humans just manage to bring their old problems along with them. Oh, and all those fancy machines that people are building? The end result of that is nuclear war.
So, if humanity is a character, we'd have to say that it's a complex, well-rounded, and totally frustrating character—one without clear motivations or even a very good understanding of what it wants out of life. You know, just like most of us.
One final thing to think about. What Bradbury does by sticking the chapters together with interchapters is highlight the interaction between the group and the individual. So, for example, he introduces the idea that old people are coming to Mars with "The Old Ones," and then looks more closely at two individuals in "The Martian."
This is cool, because it shows that there are individual differences within the group, and the role of the group vs. the individual was a hot, hot, hot topic in the 1940s. This is the time of poodle skirts and picket fences; good union jobs, the rise of the middle class, housewives, and Father Knows Best. (PSA about that link: smoking is bad for you, Shmoopers.)
Yeah, of course America wasn't really like that. the 1940s—as Bradbury knows perfectly well—were also a time of Jim Crow laws and Beatniks On the Road. But the idea of conformity was very prevalent. As Spender says, "Anything that's strange is no good to the average American" ("—And the Moon Be Still as Bright," 219).
So, think about the way a lot of these stories show individuals setting out on their own—deciding to explore Martian history, or plant trees, or build a giant and creepy not-very-fun-house. But then what happens at the end? Almost everyone decides to go back to Earth. To most of these characters, being home is still more important than "being yourself."