The Red Mail, also known as the Red Clothes, is a red tunic that appears in several games in The Legend of Zelda series. Its effects vary depending on the game.
Link obtains the Red Clothes in The Legend of Zelda: Link's Awakening DX
In The Legend of Zelda: Link's Awakening DX, Link can obtain the Red Clothes after completing the Color Dungeon—when he reaches the end, a fairy queen gives him the option to choose either the power of red for offense or the power of blue for defense. This permanently replaces Link's green Jerkin, although Link can optionally change to the Blue Clothes by going back to the fairy queen another time. The Red Clothes function mostly the same as a Piece of Power, including increasing lift speed by a fraction of a second, except walking speed is unaffected. If Link obtains a Piece of Power while wearing the Red Clothes, he does four times the usual damage to enemies. However, most bosses are not affected by the Red Clothes, and regular enemies are sent flying when struck by Link's sword, which may not always be advantageous such as during inconvenient item drops. As the whole outfit is red, it resembles the Goron Tunic from The Legend of Zelda: Ocarina of Time.
In the Nintendo Switchremake, the Red Clothes are known as the red mail, and the Fairy Queen can revert Link back to the green tunic. It also no longer changes the color of Link's sleeves.
In The Legend of Zelda: A Link Between Worlds, the Red Mail is a direct upgrade to the Blue Mail and thus appears in either Lorule Castle or Swamp Palace if the Blue Mail was skipped. It comes with green sleeves like in A Link to the Past and acts the same as in that game, but the hat is red like in Link's Awakening DX.
