In the book Allegiant, the characters have changed since the beginning  of the series. Beatrice was a small Abnegation girl who discovered she was Divergent- she could choose any faction to join. After choosing Dauntless, she had to toughen up and fight to survive, because she was facing people who had been in Dauntless their whole lives.

Tobias, a.k.a. Four has also changed since the beginning of the book. Four was an Abnegation child as well when he chose Dauntless. He did this to make sure he would be able to defend himself in later life, and to escape his abusive father. Four became an initiate instructor and eventually fell in love with Beatrice. This made him realize that there was more to life than fighting all the time.

Uriah was a Dauntless initiate that had come from amity, the faction of peace. Uriah always stepped down and let other people do whatever with him, but with help from his brother Zeke and his friends, Uriah learned to step up and fight for what he believes in.