King Boo

King Boo is the boss of the Big Boo Battle level. Defeating them unlocks Luigi as a playable character. They are a Big Boo with a crown.

Fight
In Big Boo Battle, they are found by jumping into a hole at the end of the level. The battle arena is a mirror room. When looking into the mirror, Mario turns into Luigi in the reflection. Then King Boo will appear. He appears invisible in the main room but appears in the reflection. They are damaged by being ground pound while not facing King Boo. They will then appear in the main room instead of the reflection, and must be damaged again to win the battle.

Name Error
In Super Mario 64 DS, King Boo is accidentally referred to as a Big Boo.