Mysterious Hero is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Super Secret Identity (Before the game begins, if your deck is sleeved, you may put another creature card on top of this one in the sleeve. Don't tell anyone! If this permanent is on the battlefield, you may pay to remove the card above it in the sleeve.) Hexproof Whenever this creature deals combat damage to a player, draw a card.