(I have edited your multiple questions since only one query is allowed per eNotes post.).
The man who brought Jem in was standing in a corner, leaning against the wall. He was some countryman I did not know. (Chapter 28)
Scout assumes that the man "had probably been at the pageant" and has come to their rescue when he hears their screams. Scout is partly right: He has not been at the pageant but he has probably heard the children screaming. Scout does not recognize the man because she has never seen him before. But she has fantasized about him for years, and he actually lives nearby. The man in the corner is Boo Radley, who has killed Bob Ewell after intervening in Bob's assault on the children. Only later, when her head is clearer and she gets a good look at his pale features and "timid smile", does it finally occur to her who the man is. Through her "sudden tears," Scout addresses him.
"Hey, Boo," I said. (Chapter 29)