The man who brought Jem in was standing in a corner, leaning against the wall. He was some countryman I did not know. He had probably been at the pageant, and was in the vicinity when it happened. He must have heard our screams and come running.

The man in the corner in Chapter 28 of To Kill a Mockingbird is Arthur "Boo" Radley.

After the school program as Jem and Scout walk toward their house, Jem suddenly stops. He tells his sister that he thinks he has heard something. Scout asks if he can detect what the sound is, and Jem replies, "No." But once they begin again, Jem suddenly tells Scout to be quiet. They hear nothing but the stillness of the night; therefore, Jem becomes convinced that he has heard this sound only as they are walking because when they stop, the sound also stops. Believing that their follower is "just old Cecil," Jem tells Scout, "Let's don't let him think we're hurrying." Scout shouts suddenly, "Cecil Jacobs is a big wet he-en!" There is no response from Cecil. When they still hear nothing after listening for some time, the children suspect someone else is following them because Cecil always responds to them.

As they near the oak tree in the Radley place, they hear someone who wears thick cotton pants and heavy shoes that he drags as he walks. Suddenly, this person is running "with no child's steps." Jem screams for Scout to run, but Scout cannot keep her balance and is crushed under the chicken wire of her costume. When Jem comes to rescue her, he is attacked, and Scout hears a crunching sound followed by her brother's scream. As Scout tries to hurry toward the sound of the scream, she bumps into someone who squeezes "the breath out of [her]." Unable to move, Scout hears the attacker "suddenly jerked backward and flung onto the ground." Then, she hears very heavy breathing coming from someone who staggers and coughs violently. "The man was walking heavily and unsteadily toward the road."

As Scout tries to find Jem, she feels the ground with her toes. Soon she feels someone lying on the ground. Scout tentatively asks, "Jem?" but there is no answer. With her toes, Scout touches the body. Because there is stubble on the face, Scout knows it is not her brother. Also, she smells stale whiskey. This person is Bob Ewell. As Scout tries to make her way home, she catches sight of a man passing under the street light. He walks with "the quick, snappy steps of someone carrying a heavy load." Scout sees that this load is Jem. Then she witnesses Atticus coming out the front door and assisting the man, who is Arthur Radley, as they carry Jem into the house.

Well geez, I don't know if I should answer this question because the answer is just so hugely neat-o and a really cool surprise...have you read chapter 29 yet?  In chapter 29, it gives the answer, at the very, very end.  But, I will give you some hints.

The attack happened near the large tree that, in the beginning of chapter 28, Scout said "was near the Radley lot".  The person you are asking about had somehow known that a scuffle was happening.  He had to have been close enough to hear Jem's scream as Jem's arm was broken, and close enough to be able to run to their rescue very quickly, because he gets there in time to save Scout as their attacker was squeezing the breath out of her.  He was old and large enough to fend off their attacker.  He also knew exactly who Jem and Scout were and where they lived, because he brought Jem right to their front door.

So, based on these clues that come from a close reading of chapter 28, can you guess who it might be?  Hopefully you can.  If not, skim chapter 29 really quickly; it is a very short chapter and will give the answer.  I hope this helps lead you in the right direction!