It's a secret to everybody
The Secret Moblin is a character from The Legend of Zelda. He is a Moblin that hides in various caves throughout Hyrule, and will give Rupees to Link. As most Moblins are hostile towards Link, the Secret Moblin is a rare sight as he actually helps Link, muttering to keep it a secret from the other Moblins. Like the Old Man, Old Woman and Merchant, it is unknown if the Secret Moblin is the same character, or if many exist in caves throughout Hyrule.
The Secret Moblin's quote, "IT'S A SECRET TO EVERYBODY", has become very famous among Zelda fans, and is often used in parodies of the series. Variations of the quote have even been reused in the same way by other characters in the series, such as the Lockpicker in The Legend of Zelda: A Link to the Past, aHyrulean Soldier and Princess Zelda in The Legend of Zelda: Ocarina of Time, an Old Man in The Legend of Zelda: Oracle of Ages and The Legend of Zelda: Oracle of Seasons, and Yerbal in The Legend of Zelda: Skyward Sword.