Jem is a well-developed character who, like a real human being, has a mixture of positive and negative traits. Overall he is a very good person, having been molded by Atticus.
Here are some positive personality traits of Jem Finch:
Protectiveness: A chief attribute of Jem is protectiveness towards Scout, Dill, and his father. He always wants them to be safe and well.
Imagination: At the beginning of the novel, we see Jem's vivid imagination when he is playing, especially when he is with Dill.
Introspection: Jem thinks deeply about the events that are unfolding in Maycomb during the trial. As he is entering adolescence, he finds it disturbing that the adults he looks up to are racists, but he nevertheless grapples with this reality rather than deny it.
A sense of justice: Jem is fully aware that the outcome of the trial is unjust and reacts with deep pain to the unfair guilty verdict. We can rest assured that when he grows up, he will be an agent for change.
Curiosity: Jem is always interested and engaged with what is going on in his world, especially when it comes to Boo Radley.
Respect: Jem is respectful toward his father and other adults in his life (most of the time).
Athletic: We learn early on that Jem is a football player, and we witness him being hardy and agile.
Sensible: Before he becomes moody over entering adolescence, Jem is always a person Scout relies on for advice and guidance. He knows, too, to be careful during the Halloween walk to and from the school.
A willingness to learn and grow: Jem does not let the difficult events shut him down emotionally. He is always ready to learn from circumstances. For instance, he changes his views of Mrs. Dubose once he gets to know her better.
Courage: Jem shows courage we he runs up onto the Radley porch and when he touches the side of the Radley house.
Jem also has some negative traits:
Bossiness: One of Jem's key negative traits is his tendency to boss Scout around and try to assert too much authority over her.
Moodiness: As Jem enters adolescence, he becomes increasingly emotionally volatile. Sometimes he hurts Scout by lashing out at her when she goes to him for advice.
Bad temper: Jem's moodiness can extend to anger, such as when he yells at Scout for questioning him about her teacher's racism.
Disobedience: While he is mostly obedient, Jem blatantly disobeys Atticus's orders that he leave Boo Radley alone.
Prejudice: Jem can see the racial prejudice in his town, but not until the end of the novel can he understand how badly he pre-judged Boo Radley as the bogeyman.