The MARIO is a distorted and glitchy being which resembles Mario, this entity has 6 legs, no head, black eyes with red pupils and five M's on his hat.
It resides on the abandoned office of WarioWare Inc. 
When you're transported to it's pocket dimension, you will find yourself in a distorted and red version of said office.
The MARIO let's out a loud, distorted and reversed version of "Itsy, Bitsy Spider"
Despite all of this, the entity is possibly completely harmless, having never killed anyone.