The Happy Mask Salesman is a mysterious man who appears in both of the Nintendo 64 Legend of Zelda games. Although he played a small role in the Legend of Zelda: Ocarina of Time, he had more significance in Majora's Mask.
He is the owner of the Happy Mask Shop, where you can rent the masks for various quests as Young Link.
He is introduced in the beginning of the game when you enter Termina, and he offers to return you to normal if you retrieve the precious item that was stolen from him, the Ocarina of Time. He explains to you that he is a busy man and must leave within three days, but he does not tell you that at the end of those three days, the moon will come crashing down into Termina. After you retrieve the Ocarina of Time, he teaches you the Song of Healing, which you will be using later in the game. He then tells you the story of Majora's Mask, which was also stolen from him. He tells you that if you do not return it in three days, you will meet a terrible fate. After you defeat Majora and retrieve the mask, he will say a few words and disappear as mysteriously as he came.