1. The kingdom's greatest hero is a dumpy plumber
Meet Mario Mario (yes, that's his last name), the champion of the Mushroom Kingdom. When evil strikes, the princess is kidnapped, or the world is under siege, it's up to... him. There are no knights, no soldiers, no anything - just an Italian plumber who can jump real good. Don't be fooled, though, Mario isn't the the best jumper in the kingdom - his brother can jump a little higher, leaving Mario with the esteemed title of "best at nothing."
Despite his shortcomings, he is the only thing that stands between the Mushroom Kingdom and complete and utter chaos. Without him, Peach is still in Bowser's clutches. Without him, Tatanga still has Daisy in a cell. Without him, the entire kingdom is in ruins under Fawful's awful rule. It's never explained why he is the one they put in charge of everything, but the Hand of the King he is, dutied with the protection of the realm. He has no powers, he has no special abilities, and he can barely put together sentences. But, regardless, he's the hero they need, even if he isn't necessarily the one they want.
And we love him for it.