The Master Chief - not to be confused with the Master Chef - is you, your alter ego in Halo. For details about all of the things which make you badass, you are advised to read The Fall of Reach, the Halo prequel novel by Eric Nylund. There's far too much information to supply here. In short, you're the biggest, baddest armored Covenant-smacker in all of human territory. Now, go read the book.