What are some physical characteristics of Jesse Aarons?
As Chapter 1 begins, we learn that Jesse likes to dress in overalls without a shirt underneath when he's going running, so we know he dresses a little on the nerdy side, but that's probably just normal clothing for kids who work on farms in Jesse's rural area.
The narrator gives us a quick description of what Jesse looks like in Chapter 1:
"His straw-colored hair flapped hard against his forehead, and his arms and legs flew out every which way. He had never learned to run properly, but he was long-legged for a ten-year-old, and no one had more grit than he."
So, we know that his hair is the color of straw, which is light blonde. He's only ten years old, but he has long legs, and his legs and arms kind of flap out everywhere while he's running--so we also know that he's gangly, or kind of in an awkward stage of his pre-adolescence. Picture a skinny kid with long arms and long legs, who's probably going to grow more muscular soon because he gets so much physical exercise, and that's Jesse.
One more physical trait is worth mentioning: Jess's body is often "tired and tensed-up," as we learn in Chapter 2. His family life is stressful, and he lives in poverty. The only kind of escape he seems to have at the beginning of the story is drawing, so he probably appears relaxed and happy when he's doing that, but otherwise, his posture is probably rigid, showing his nervous disposition.