Boo (Mario Party 4)
As shown in the game's end credits, Boo appeared as a generic member of his species before coming across the Party Cube emerging from a Warp Pipe. Together with Toad, Shy Guy, Koopa Troopa and Goomba, they decorated the Party Cube with drawings of themselves before sending an envelope of invitations to Mario and his friends. After dressing up in costumes to match their board themes, they enter the Party Cube and head to find Mario and his friends.
In the game's story mode, Boo, along with the other hosts, promises the player a present should they complete the map. However, after the player beats the map, Boo reveals a catch to the present, claiming the player must beat him in a minigame before they get the present. After defeating him in a game of Mystic Match 'Em, Boo will give the player their present.
According to the Mario Party 4 Prima Guide, Boo is portrayed as a mischievous character, intentionally trying to ruin the party for Mario and his friends by scaring them at any given opportunity. This is strongly shown by his board's theme, which he claims is meant to scare anyone who entered it. Much like the speech of other Boo related characters, Boo constantly gives off mysterious esque laughter. He also wears a wizard's hat and cape as his costume choice.