Waluigi is an important character in the series, and is acted by Doug Orofino. He is Wario's brother. He doesn't have a mustache because it is supposed to be a younger version of the popular Nintendo character. He is also known for being the boyfriend of Walgina, a made-up character.
In the series, Waluigi is a foe, and helps Wario bring the Mario Brothers back to the Mushroom Kingdom for Bowser. He also has a girlfriend, Walgina.
In the movie, Waluigi becomes friends with the Mario Brothers. Waluigi doesn't have that big of a role in the movie. He is seen in ACT I Part 1 dancing with Mario, Luigi, and Wario. He then doesn't make an appearance again until ACT II Part 5, where he is seen talking with Merlin near the end. He will probably make another appearance, but no one knows, as Part 6 hasn't been released yet.