Most of the information in the story is about Mathilde. The first character revelation is in the very first sentence.
The girl was one of those pretty and charming young creatures who sometimes are born, as if by a slip of fate, into a family of clerks. (p. 1)
Here it is revealed that Mathilde is pretty, but not happy with her station in life. She is poor, and wants to be rich.
The second character revelation about Mathilde is indirect characterization by the way she reacts to her husband’s ball invitation. He worked hard to get her the invitation to the popular party, thinking she would be pleased.
He stopped, distracted, seeing that his wife was weeping. Two great tears ran slowly from the corners of her eyes toward the corners of her mouth. (p. 2)
Mathilde is not pleased. She does not want to go to the ball because she does not think she has good enough clothes and jewels. This tells us that Mathilde is a proud and vain person, and very class-conscious.
Finally, when Mathilde loses the jewel she borrows from a friend we learn something else about her—she is determined. Her pride is so great that rather than tell her friend the truth about losing it, she lies and says it broke. She then goes into debt replacing it, and works ten years trying to pay off the debt. Yet she does it.
And dressed like a woman of the people, she went to the fruiterer, the grocer, the butcher, a basket on her arm, bargaining, meeting with impertinence, defending her miserable money, sou by sou. (p. 6)
In the end, Madame Loisel has perhaps more gumption than we thought, but she brings her hardship on herself with her continued pride.