Boo Radley is a Heroic Albino.When Scout finally meets him in person, he is described as being ghost-like, with very pale hair and skin. Scout, not knowing what albino is, would just assume this is because he stays inside all the time, rather than Radley being born like that. This would also explain why Radley is familiar with the doctor; he probably would have medical problems which often come with albinism like bad eyesight. It might also be partly why he lives such a reclusive life, as if he was an albino he would be easily identifible in a small town and rumours could spread easily about him, (especially in 1930's Alabama).
Bob's kids have different mothers.Although Bob Ewell sexually abused Mayella, the story has never mentioned her being pregnant. So maybe she isn't the mother of the kids. Bob maybe had kids with various prostitutes.
Boo Radley is a War VeteranOne of the gifts that Jem and Scout find left behind by Boo happens to be a war medal. Assuming Boo Radley is in his late 20s or early 30s he would have been old enough to have fought in World War I. If he is a war veteran it would explain why his behavior is so odd, he is suffering from Shell Shock or what we now call Post-Traumatic Stress Disorder. Boo Radley giving up a war medal to these children shows how much he cares about them, he's reaching out to them with a symbol of the experiences that caused his trauma in the first place.