It's been a while since I read this book, but I would venture a guess that this would be the parachutist. He came from up in the air, and air surrounds the boys--as does the war, even though they are unable to perceive from it their cozy little island. Air is everywhere. Evil is also, if you choose to be tempted to follow the dark side of humankind as opposed to doing what you know is the right thing as dictated by conscience.
Hope this helps you a bit. Reread that chapter with what I've said in mind, and see what you can find.