There was a sudden bright explosion and a corkscrew trail across the sky; then darkness again and stars. There was a speck above the island, a figure dropping swiftly beneath a parachute, a figure that hung with dangling limbs.
This is how the parachutist arrives into the island: dead, and following some sort of explosion. We don't know precisely who he is, or how he's arrived: has he been shot out of his plane? Has he been evacuated from a failing plane?
Either way, the key thing he represents from the adult world is the war which caused the boys to get stranded on the island in the first place (their plane, Golding implies, was shot down) and which has the naval officer who finds them at the end on the seas. War, and the violence of man to man is what the parachutist represents: both as a serviceman himself, and in his assigned (wrong!) role as the beast - which catalyses all the violence which breaks out on the island.