The Happy Mask Salesman is a character that appears in both Legend Of Zelda games for the Nintendo 64, Ocarina Of Time and Majora's Mask. In Ocarina Of Time, he runs a shop in Hyrule Town where Link can obtain masks he can sell to various people to earn money for the salesman. In Majora's Mask, he was the owner of Majora's Mask until the Skull Kid mugged him and took it from him, using it (or being used by it) to create all sorts of trouble in the land of Termina, including causing its moon to crash down on it. In that game, the Happy Mask Salesman implored Link to retrieve the stolen mask from the Skull Kid. The Happy Mask Salesman can identify any mask you how him, though will not be satisfied until he sees the Majoa's Mask.
Plus, he smiles WAAAAAAAAYYYY too much. I think he likes it when you die.