What is some textual evidence to support a claim that Boo is protective?
In chapter 28, Boo Radley saves Jem from Bob Ewell. We find out later who the he is in the following words, but this quote demonstrates Scout's recognition that someone just saved Jem from a fight with Bob Ewell:
A man was passing under it. The man was walikgin with the staccato steps of someone carrying a load too heavy for him. He was going around the corner... carrying Jem. Jem's arm was dangling crazily in front of him.
Later, in chapter 30, Heck Tate talks about Boo Radley and certainly characterizes him as a protective man trying to do the right thing for others:
I never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did.
This shows that Boo did what he could to protect the kids by preventing the crime of murder that was sure to happen to them with the intentions Bob Ewell had.
In chapter 31, Scout says of Boo:
He gave us two soap dolls, a broken watch and chain... and our lives. But neighbors give in return.
Scout was sad because Boo had protected her very life, but she never felt as if she did anything for him.
You may also consider the blanket Boo placed on her unknowingly in chapter 8.