What three personality traits of Jem Finch are revealed in Chapter 15 of Harper Lee's To Kill a Mockingbird?

Many of Jem's best personality traits are revealed in Chapter 15 of Harper Lee's To Kill a Mockingbird. One of the most important traits revealed is protectiveness. More specifically, he demonstrates devotion to protecting his own, such as his father. While demonstrating devotion to protect, he also demonstrates bravery and sheer stubbornness.

Jem very easily senses danger brewing when a crowd of men gather to talk to Atticus in the front lawn one evening. Though Atticus insists he was not being and would not be threatened by a mob, Jem continues to listen to his instincts telling him danger was near. Jem becomes so uneasy that, after Atticus leaves the house at a suspicious time and carrying a suspicious light bulb attached to an extension cord, Jem refuses to go to bed and instead dresses to leave the house as well, telling Scout, "Scout ... I'm scared. ... Scared about Atticus. Somebody might hurt him." After that, Jem, Scout, and Dill leave to go downtown to try and see what Atticus is up to.

Once Jem sees that a mob has approached Atticus as he guards the jail door and Scout makes her presence known, Jem displays an urgent desire to protect and a great deal of bravery by refusing to go home. Four times Atticus orders and begs Jem to go home, but all four times Jem refuses, which also shows his determination and stubbornness. Though Atticus had feared for the children's safety, after Scout saves the day with her naive friendly conversation with Walter Cunningham, Atticus expresses his appreciation of Jem's determination to protect, bravery, and stubbornness, as Scout describes in the following:

Atticus and Jem were well ahead of us, and I assumed that Atticus was giving him hell for not going home, but I was wrong. As they passed under a streetlight, Atticus reached out and massaged Jem's hair, his one gesture of affection. (Ch. 15)

Hence, in assertively striving to protect Atticus, Jem demonstrates he possesses the traits of protectiveness, bravery, and stubbornness, all traits he shares with his father, and all traits his father is very proud to learn Jem possesses.