When Jem finds his pants folded and mended, he asks Scout, "Can't anybody tell what I'm gonna do lest they know me, can they, Scout?" [italics added]

Who is it who knows Jem and mended his pants?

Jem loses his pants while he was trying to escape from the Radley's house at night. He was accompanied by Jill and scout. His pants were caught by the fence of the house while he was running away.

Boo Radley was the person responsible for mending Jem's pants. He mended it and then placed it neatly across the fence.