Since Scout is narrating the book, every sentence could potentially describe her. So, let me proceed by giving three adjectives that describe Scout with a quote to back it up. Scout is: precocious, tough, and empathetic.
First, Scout is precocious. She is well ahead of her peers in intellectual matters. Moreover, she is incredibly curious. For instance, when she went to school, she was already reading. More than that, her reading level was very high. Here is what Jem says about Scout's reading ability to Dill:
“Shoot no wonder, then,” said Jem, jerking his thumb at me. “Scout yonder’s been readin‘ ever since she was born, and she ain’t even started to school yet. You look right puny for goin’ on seven.”
Scout is also tough. She is a veritable tomboy. For example, when she feels that Walter got her in trouble, she beat him up.
Catching Walter Cunningham in the schoolyard gave me some pleasure, but when I was rubbing his nose in the dirt Jem came by and told me to stop. “You’re bigger’n he is,” he said.
“He’s as old as you, nearly,” I said. “He made me start off on the wrong foot."
“Let him go, Scout.
Finally, it took a while, but in the end, Scout learned how to see things from the perspective of others. At the end of the novel, Atticus thought he would have a difficult time trying to explain that Bob Ewell fell on his knife, but Scout was way ahead of Atticus. To protect Boo Radley was a good thing. For her it was protecting a mockingbird, a point that Atticus probably did not consider. Here is the quote:
Atticus disengaged himself and looked at me. “What do you mean?” “Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?”