In the novel, each of the girls struggles with a fault. For Jo, her fault, or "burden," is her hot temper. When she gets angry, people know it. She lashes out and has a hard time forgiving.
Her anger gets the best of her in chapter 8 when Amy burns a manuscript that Jo has been working on. Jo flares up:
Jo's hot temper mastered her, and she shook Amy till her teeth chattered in her head, crying in a passion of grief and anger.
Jo feels she can never forgive Amy for what she has done. It is only when Amy almost drowns that Jo comes to her senses and remembers that she loves her sister. This chapter also illustrates another of Jo's traits: her passion for writing.
Another important trait in Jo is a willingness to sacrifice herself for the good of others. She lives with integrity and does what she believes is right, even if she has to suffer for it. For example, she works as a paid companion for her Aunt March, even though it drives her crazy, because the money is needed for the family finances. She turns down Laurie's marriage proposal, although it half breaks her heart, because she fears that marriage will make them both miserable. The loneliness that she experiences afterwards is hard to bear, but Jo has done the right thing.
Jo also shows the traits of pride, independence, and resourcefulness. In chapter 15, she sells her hair to raise $25 for her mother to take the train to Washington to be with their father. She does this because she doesn't want Marmee to have to borrow the money from Aunt March. As Jo says of the cash she hands to her mother,
I didn't beg, borrow, or steal it. I earned it, and I don't think you'll blame me, for I only sold what was my own.