We learn quite a bit about Jean Louise throughout the novel, not only from other characters who comment on her traits, but also from the narrator, and by examining her actions.
In Ch. 1 we begin to have a picture painted of a fairly stubborn woman.
"She was a person who, when confronted with an easy way out, always took the hard way" (15).
In addition to this tidbit, we see her bantering right away with her long-distance boyfriend, Hank, when he comes to pick her up from the train station. They play-argue back and forth the entire way to her house. From the exchange we see that she is very independent and has no intention of marrying him, or anyone. This is not only independent of her, it is a unique choice for a woman in the 1950's.
By Ch.2 we understand that Jean Louise argues with people easily and fairly often. She is not one to bite her tongue. Upon coming home and already being at odds with Aunt Alexandra, Atticus says to Jean Louise,
"Apologize to your aunt. Don't start a row the minute you get home" 22).
As the book goes on we also learn she is playful and competitive, mostly with men. We see her play different games with Henry, Atticus, and Uncle Jack. In Ch. 2 Atticus and Jean Louise plan to go play golf, and in Ch. 5 she races Henry to the landing and then jumps into the river.
One of her strongest and most admirable traits is probably her integrity. This is demonstrated throughout the book, but most obviously in Ch. 17 where her and Atticus discuss race relations. Here she stands her ground with him, even as he expresses beliefs that are completely counter to everything she grew up believing. She tells him this - that in raising her the way he did, he actually was part of a lie. She thought he was a fair man and yet he is a bigot like so many of the people she thought he was against. It must be very hard for her to stand up to this man whom she so loves and respects and tell him how wrong she thinks he is. This shows that her own integrity and belief in what is right and wrong actually trumps the relationship she has with her father at that moment.