The ghost is a character that a player can turn into in Cruise Story if they own the Be the Ghost Gamepass. When a player with that gamepass first joins a lobby they'll be asked whether they want to activate their gamepass. If they press "Yes" then they will have a green aura as well as a unique title. The player also gets a sword in their starting inventory.
By activating the gamepass and playing a round of Cruise, there is no way to get any monster ending, but you do get some cool stuff and a green new look!