The Happy Mask Salesman, also known as the HMS, is a character who makes his appearance in Zelda: Ocarina of Time and Zelda: Majora's Mask. He is a disturbingly always happy guy to many people who have played these games and encountered him. He is not feared as an enemy to some, but is dreaded to others.
He also makes in an appearance in the BEN DROWNED (Haunted Majora's Mask) creepypasta written by Alex Hall, AKA Jadusable. He appears to team up with BEN later in the story and is one of the first characters, except the stature, that he first notices is out of place and is making strange and irregular appearances in places or making random screen cuts. But no matter what, wears the same grin.
People have theories that the HMS was "the father" BEN refers to in a dialoug in the story. In fanfics and other stories, he is the one who killed BEN and remains with him in the afterworld.