Much like regular boos he is round and white allthough he can appear pink as well.
Mairo Party 5 marked the first apperance of the playable Boo. He is the first newcomer in the game alongside Toad and Koopa Kid. His role in previous games, stealing players stars and coins, was given to Chain Chomp, and later Pink Boo. He reappears as playable character in Mario Party 6, 7, 8, Island Tour and Super Mario Party.
In Mario Party 6, after Brighton and Twila have an argument, thus causing the world's day-night system to fall apart, Boo helps the two to cheer up by collecting stars to fill in the Star Bank, which restores peace to the land.
In Mario Party 8, Boo is one of the characters that can be chosen to get the Star Rod back from Bowser after he steals it from MC Ballyhoo.
In Mario Party: Island Tour, Boo can be chosen to save Party Island from Bowser's controls. Although he is unable to truly defeat Bowser, he is able to defeat a majority of his bubble clones.