Mario is a fictional character, created by Japanese game designer Shigeru Miyamoto. Serving as Nintendo's mascot and the protagonist of his series, Mario has appeared in over 200 video games since his creation. Pretty much everybody knows this awesome, short, mushroom-eating, Italian plumber!
In Mari0, he is pretty much the same as in his source game, except for the Portal Gun he holds in his hands. While Mario is the only playable character in 1.6 and below, more are available in Mari0: SE. Additionally, mappacks in Mari0: SE can control when Mario has the portal gun.
The game itself does not provide a reason for him having the gun. However, some user-created mappacks do.