King Boo is the main antagonist of the Luigi's Mansion series, and a recurring villain in the Mario Bros. series, as he was the one responsible of kidnapping Mario in a painting.
And he is voiced by Charles Halford in the Luigi's Mansion series in the English dub (But as it turns out that the Luigi's Mansion version is the real deal) and he will be voiced by Jouji Nakata in the Japanese dub, just like Bowser.
His original series form is voiced by Matthew Mercer in the English dub and he will be voiced by Ryusei Nakao in the Japanese dub.
And his Super Mario Sunshine form is voiced by Frank Welker in the English dub and he will be voiced by Kappei Yamaguchi in the Japanese dub.