Quentin

The Quentin in the movie was pretty much a combination of two individual Rocket Boys in the book (more on that later). The Quentin in the book was also smart and somewhat more knowledgeable about all things scientific than Homer, and he did have a lot of common sense (but he was not as intrepid/resourceful as Homer; i.e., "thinking outside the box", although he did have those moments, like the bit about making the rocket thin and long). And it was made clear that he was the most eccentric in his mannerisms and speaking affect. But even with all that he was maybe the most self-assured of the Rocket Boys besides Roy Lee. While Quentin's family was also more destitute than the others and lived in the outskirts of town where the people too poor for even Coalwood lived, he was always upbeat and never really worried about the others knowing his situation. It's probably because of his outlook in addition to his talents that the others learned to get along with him. I agree, he's not as obnoxious in the book as sometimes in the movie.

The aspect of his movie character where he's ashamed of his living condition was derived from the other Rocket Boy in the book—Billy Rose. Billy was just as poor as Quentin (relative to the other Rocket Boys, none of them had rich families). And Billy's real-life dad was the alcoholic who abused him as described in the book, so part that of his actual life was also combined into Roy Lee's character instead of Quentin in the movie. (The real Roy Lee did have a step-father because his dad really did die in the mine, but in the book that step-father never beat him up; I can't remember in the book if Homer's father had some other sort of grudge/conflict related to work in the mine with Roy Lee's step-dad, but it could be the case).

I don't know if they were trying to imply that Quentin's father was absent from his life somehow by not having him portrayed at all in the movie, but in the book I believe he had both parents. Also in the book, he wound-up staying in Homer's house a lot, mainly because he lived farther away than the other Rocket Boys, so it became a convenient time-saver by staying with Homer. He also appreciated Homer's mom feeding him and mending his clothes, although it was made clear that it wasn't from his own parents inability to feed clothe him adequately. It's just that was a little bit better than what he usually experienced at home.

PS: In real life there were also two other Rocket Boys combined into one movie character: 1) Jimmy O'Dell Carroll, who, along with Roy Lee, was Homer's friend even before high school and joining the Rocket Boys. His dad didn't die in the mine either. In fact, his dad was the town's garbage collector and junk-man, so the Rocket Boys often were able to use some of the junk and scrap he was able to save for their "rocketeering" endeavors. 2) Sherman Siers, who was the one with the limp from polio and is the only Rocket Boy who isn't alive (he died in the 1970s from a heart attack while he was still in his thirties).

But like in the movie, Homer's friends before forming the Rocket boys were just Roy Lee and O'Dell. They were the only ones from Coalwood. All the other Rocket Boys lived nearby and all went to the same regional high school that served multiple towns.