In Occarina of Time he sells a limited range of masks from the Happy Mask Shop in Hyrule town square. He lends you masks so that you can sell them for him - if you can sell enough you can obtain the Mask of Truth for yourself. Althaugh usefull, the masks are only a minor detail of Link's first outing on the N64

In Majora's Mask, as the name would suggest, masks and the Happy Mask Man make a sinister return. In the sequel, he seems slightly twisted as if the evil mask he once owned had a powerfull effect on him. When the Skull Kid steals the most powerfull magic mask of all he travels to the doomed world of Termina in order to recover Majora's Mask.