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!