Ok let's set the record straight... her name is spelled SHELLIE MAY.
And to me, she's Duffy's best friend (though some like to think of her as his "girlfriend"). She is only typically seen at Tokyo DisneySea, though earlier this year she made a surprise appearance at Disneyland Paris for a special private event that was not open to the general public. She hasn't been seen at the Paris park since then though.
And her backstory is as follows (which takes place after Duffy's backstory).... After Mickey & Duffy came back from their adventures at sea, Minnie decided to make Duffy a friend of his own. So she made a girl bear. Duffy was happy to have a friend of his own and gave her a heart-shaped shell he found... that's how she got the name Shellie May.
And yes Shellie May is as popular as Duffy at TDS. They're even changing the "My Friend Duffy" show at the Cape Cod Cookout restaurant to incorporate Shellie May.
As a side note, I just do not get all the hate toward Duffy some of you have here in the States. He's just a little teddy bear that has done NOTHING to you. Just because he's not associated with a movie or a TV show, Americans can't seem to relate to him. SO WHAT? The biggest problem is how he's presented in the US parks. Yeah I won't deny Disney saw his popularity in Japan and saw the dollar signs, thinking that same success would be translated to our side of the pond. Yeah... no. Over at TDS, Duffy fits in with the area of the park he resides in. They even state Duffy's "hometown" is Cape Cod. Over here he's just plunked into a generic meet & greet location with no rhyme or reason. In Japan, he has a fully-themed meet & greet area with a lot of rich details that add to his backstory. And you know the US parks aren't going to do something like that. it's a cultural thing. The Japanese like characters that are cute, regardless if they're associated with a movie or a tv show or what have you. But geez... the hate is really ridiculous because, you know, it's a TEDDY BEAR.
To me, Duffy is a fun, cuddly travel companion. I'm no kid (pushing 40 here) and I take Duffy with me whenever I go to the parks or on special adventures. I might get a weird look from time to time when someone sees him poking out of my backpack/bag but most times I get "Oh your bear is so cute!" I've also noticed most of the Duffy hate spawns from the west coast. Over on the east coast/WDW, he seems to be more accepted. I visit WDW once a month (AP holder - I live 1.5 hours away) and I always have guests and CM's fawning over my Duffy. When I was at the parks a little over a week ago, I actually saw several of all ages with Duffys or buying Duffys.