Let me preface this by saying this dude is one creepy SOB. I have spent many playthroughs trying to avoid seeing his face. Sure, he is calm and helpful on the surface, but once you piss him off, he is a very dark man.
Starting off innocently enough as a simple mask salesman, he serves as Link's way to access certain areas to progress in the game. He's nice and cheers Link on as you sell the masks for him, but if you return to shop without his money, he becomes livid and kicks you out. After the town is destroyed when Link is an adult, you no longer see him.
That is until Majora's Mask, where he is no longer just the simple salesman but rather a key point in the game.
Said to be a traveler between dimensions, he has a knack for creeping most people out. Besides his love for masks, most of his background is rather shaded. The masks he sells are ones he's collected while travelling between dimensions. He does help Link gain some extra rupees as well as some nightmares.
I can only say I didn't rate him any higher because he didn't become much of a salesman after the few missions he gives to Link. He was limited to selling masks, and didn't offer any rare reward items beside the Mask of Truth in Ocarina. I placed him on the list in general because I find him an interesting shopkeeper. (And I'd rather not hear him say "You have met with a terrible fate" in my sleep for not placing him.)
Image Source: ZeldaWiki