King Boo is an enemy and boss from the Mario series and the main antagonist of the Luigi's Mansion series.
King Boo is slightly larger than the average Boo. In the first game he ever appeared, he had menacing red eyes, purple ruby on top of his head, black eyelashes, sharp fangs, and blue tongue. His modern look is similar to his original look, but wears a golden crown that resembles Princess Peach's, less menacing-looking black eyes, and his tongue is now pinkish-red. His fangs are still the same as original.
His first game was in Luigi's Mansionas the main antagonist, where he traps Mario in a painting, and plans to do so with Luigi. Luigi has to defeat King Boo with his Poltergust 3000, which will suck up any Boo. In the final fight with King Boo, he is actually piloting a giant Bowser mecha, and when he comes out, that's your chance to harm him.
In Luigi's Mansion, he is the size of a regular Boo, if not slightly bigger, has a bigger mouth with vampire like fangs, and a ruby crown on his head, but in Super Mario Sunshine he is much larger and wider then he was in Luigi's Mansion and has tired eyes, and a huge, long tongue coming out of his mouth. His battle is in Hotel Delfino's Casino. In Mario Kart: Double Dash and other games he looks more like he did in Luigi's Mansion, only with different colored eyes and a different crown.
He also appears in Luigi's Mansion: Dark Moon once again as the main antagonist, where he once again traps Mario in a painting, as well as shattering the Dark Moon in evershade valley. His appearance is more similar to his Luigi's Mansion appearance, except he still has a full row of teeth like his modern appearance instead of his vampire like fangs in Luigi's Mansion, he also speaks with his sped up voice in the modern Mario games.
King Boo comes out of a Bowser suit at the climax of Luigi's Mansion
King Boo on the Luigi's Mansion 2 Promotional poster