coaster buddy
Someone who you would go on a roller coaster with in order to keep you company. Without a roller coaster buddy, it would have to be pretty embarrassing to be the only one in your row on the coaster train. It is a known fact that single people want roller coaster buddies.
Joey: Will you be my coaster buddy?
Rachel: Sure, I'll keep you company on the roller coaster.
Rachel: Sure, I'll keep you company on the roller coaster.