Bowser

Bowser is the a villian in the series. He doesn't appear too much in many videos, mostly the series. He is a koopa turtle thingy that plans to rule the universe, but cannot do it due to Mario and  Luigi. Bowser has hired many people, like to say: Boo,  Petey Piranha,  The Bulldog, etc.

Death
Bowser had died in the end of Luigi's Shorts, returning as a ghost... a more powerful Bowser n Mario and Luigi World Adbenture