At the beginning of To Kill a Mockingbird, the most predominant character trait of Boo Radley is that he is mysterious. Much of what the audience learns about him (directly) is from gossip, here-say, and the opinions of the main characters, who are children and who have almost glorified Boo into a monster of sorts out of boredom.
However, there are several personality traits that can be inferred by using indirect characterization. It is true that Boo Radley is a character who keeps to himself. This is evidenced by the fact that the children have never seen him. "Nobody knew what form of intimidation Mr. Radley employed to keep Boo out of sight" (11) but this fact keeps the children guessing at his physical appearance.
It can also be inferred that Boo Radley has an innocent interest in the children. This is evidenced by the gifts he leaves in the knot hole in the tree in chapter 7. The soap carvings of the boy and girl who resemble Scout and Jem are almost like a peace offering showing that he is not a monster. The children's opinion of him begin to change.
Finally, it can be inferred that Boo Radley is not only interested in the children, but watching out for them as well. On the night when Jem, Scout, and Dill sneak into the Radley's back yard and Jem loses his pants, he goes back for them only to find them neatly folded and sewn up "not like a lady sewed 'em" (58). Jem knows before anyone else that Boo Radley is seeking a method to communicate with them, and though he remains a character surrounded by mystery, it seems clear that he is not harmful.