Red Mail (Link's Awakening)
"You've got the Red Clothes! Your body is full of energy!"
The Red Mail - known as the Red Clothes in Link's Awakening DX - is an optional item available only in Link's Awakening DX and Link's Awakening for Nintendo Switch, not the original Game Boy release. Wearing it doubles Link's offensive powers, doubling the damage dealt by Link's attacks.
On completing the Color Dungeon, Link finds himself in a room with Koholint Island's Fairy Queen, who will offer him the choice between the blue mail which doubles defence, or the red mail which doubles attack. Whichever he chooses, he can return to the Fairy Queen at a later time to switch to the other option.