King Boo is the ultimate villain in both Luigi's Mansion 1 and 2, and is also present in Luigi's Mansion 3 as seen in the trailer from E3**. He is Luigi's nemesis, and goes through great lengths to terrorize him and his brother Mario. Luigi has access to the only thing that can stop him, the mighty Poltergust and all of its evolutions. This model is ripped straight from the original game in 2001, T-Posed.
"I am not afraid of you, fool! I fear only that infernal Poltergust 3000 you carry on your back! Stupid machine!" -King Boo, Luigi's Mansion.
**Here's to hoping that Waluigi is somehow related to the plot! :)