Arguably, this kid is responsible for the entire crisis of the movie. Mitch is a neighbor kid who lives next door to Kevin, and he's nosy and full of curiosity. While the van driver waits to pick up the McCallisters, Mitch pesters him with questions:
MITCH: How fast does this thing go? Does it have automatic transmission? Does it have four-wheel drive?
DRIVER: Look, I told you before kid: don't bother me. Now beat it!
But Mitch doesn't beat it. He hangs around and screws up the head-count—Kevin's older sister Heather accidentally counts Mitch, making the family think that all the kids are accounted for.
Thanks, Mitch. Your annoying meddling just generated a classic family comedy.