— Big Boo to Mario
Big Boo is an enemy that first appeared in the Ghost Houses of Super Mario World. They are the same as regular Boos, but are larger in size. One Big Boo made an appearance in the popular Bowser's Kingdom show.
Big Boo first appears in Episode 3, where he is seen among the many minions in the World 3 Castle. When Mario eventually reaches him, Big Boo states that he's going to get him once Mario stops looking at him. Much to Big Boo's dismay, Mario dodges him by walking backwards while facing him.
In Episode 8, Big Boo appears as the Spooky Speedster after Jeff transforms into a Boo. He tempts Jeff into a race for a star, but Hal states they're not in the mood. Suddenly, Luigi appears and sucks up Big Boo with his Poltergust.
Big Boo makes a reappearance in the Bowser's Kingdom Movie, where he is seen in the Ghost House that Hal (disguised as Luigi) and Princess Peach visit while searching for Jeff (disguised as Mario). After appearing, Peach becomes scared and tells Hal to kill him, but Hal states to her that ghosts can't be killed. Big Boo later disappears after Petey Piranha breaks in and throws up on Hal and Peach.
In Episode 666, Big Boo is one of the few survivors in the castle on the Doughnut Plains (most likely because he's a ghost). After Geno finishes his story of how the zombie invasion occurred, Big Boo states it's because "the devil was upon them", in the form of Ganon. He then states he's only joking and that the rest of the survivors are "all gonna die", before disappearing in a flash.