Mario doesn't like you he will bring pikachu and destroy mailboxes there is a cave of invincibility by the way no one seems to know where it is except me its awesome in that cave there are other things like unlimited mushroom power ups and things that will let you win the entire game in a week or so. And if you do all the games once without getting killed it will show you where the cave is in case you play the game again but you can find it in the first level first world. Its through a tube and hint its right in front of your beep eyes! I was so happy because I found out you can only get it if you didn't do any other levels or worlds yet.
You can't unless you have super smash bros brawl melee for wii
I don't think unlocking him is possible