Meta Knight is a warrior of legendary power who "fights for destiny". He runs around, and kills powerful bad guys, because he is awesome like that. He can slay even the most dangerous villains.
Meta Knight was born on Mars. He was recruited in the Martian army at the age of three. They trained him for twenty years, and he became one of the best agents they had.
During the Nonsense War, the Meta Knight was flown to Earth to kill people with his legendary fighting skills. He crashed into Earth, and started killing. His General told him to "STAND DOWN SOLDIER!" So, the Meta Knight turned his sword on the general.
The Meta Knight was dishonorably removed from the Martian military, and he was told never to return to Mars, or they would blow him up. He chose to live the weird Earth planet after that. He now plays cards with Chuck Norris, and has almost won once.
But then he was revived by Googolplex (Who didn't mind Meta Knight wearing his mask at all times), and he told him: "Want to get revenge on both King Dedede and Kirby? Join me and my empire, and you'll be safe and sound!" Meta Knight agreed, and joined the Satanist Empire.