Wario is an important character in the series and is acted by Matthew Thomas Provencal. He is Waluigi's brother. He doesn't have a mustache because he is supposed to be a younger version of the popular Nintendo character. He first appeared in Episode 1 and has played an important role in the plot since then.
Wario was mainly a foe to the Mario Brothers in the series, and was working for Bowser. He wanted Wario to bring the Mario Brothers back to the Mushroom Kingdom from the Real World. Waluigi joined Wario, and helped him reach this goal. They failed, and they quit working for Bowser.
In the Movie, Wario and Waluigi became friends with the Mario Brothers. This shows most in Part 1 of ACT I, when the 4 of them are dancing. Wario later fights and beats Shadow Mario using the Truth Stone. He then gives the Truth Stone back to Merlin. Later, Wario helps Cocount turn back into a human.