According to Miss Maudie, she knew Arthur Radley Jr. as a boy and
"He always spoke nicely to me, no matter what folks said he did. Spoke as nicely as he knew how."
We do know the basic facts of Boo's first run-in with the law. He had been hanging around the Cunningham boys, and they were arrested for
... disorderly conduct, disturbing the peace, assault and battery, and using abusive and profane language in the presence and hearing of a female.
The boys had stolen a "flivver" (probably on old car) and locked Mr. Conner, "Maycomb's ancient beadle" (a minor church official), in the courthouse outhouse. The other boys were sentenced to the state industrial school, but Boo's father wouldn't allow that for his son. He agreed to deal with Boo himself, and the judge released young Arthur into his father's custody.
... Mr. Radley's boy was not seen again for fifteen years.
Many years later, according to Miss Stephanie Crawford (whose story may contain some errors or exaggerations), Boo stabbed his father in the leg with a pair of scissors. Boo was jailed in the courthouse basement since
... the sheriff hadn't the heart to put him in jail alongside Negroes.
Once again, Boo was released into his father's custody, and again he became a prisoner in his own house.
We can assume it was Boo who left the children gifts in the knothole of the Radley oak; who mended Jem's pants after his visit to the Radley's back porch; and who placed a blanket on Scout's shoulders on the night of Miss Maudie's house fire. The rest of the stories about Boo--killing pets and peering in windows at night--may or not be true. His only visible appearance in the novel comes when Scout sees him in the shadows of Jem's room after the attack by Bob Ewell; and we can assume it was Boo who killed Bob, protecting the children he had been watching grow up from within the confines of his house.