Someone who is innocent, good, kind and naive. The difference being being just a regular, run o' the mill nice guy and a white shirter is WHITE SHIRTERS ARE NOT TO BE CORRUPTED. Communities of peeps unite to keep the white shirters blissfully unawares of the nasty reality of life.
'MAN THAT BITCH IS HOTT'
'be careful what you say to her, she's a white shirter'