Russ mentions Manic Pixie Dream Girls. I love them! I even know some in real life. (Hi Sally, hi Bettina.)
Unlike what they say at the link, I do think they exist in real life. It's a mis-definition to say they are "always happy". Look at Natalie Portman's character in Garden State, she is far from happy all the time.
But it may seem that way, and I think that's the reason they are rare in real life: one of the qualities of MPDGs is that they seem to float a little bit above life, it does seem to affect them as seriously as it does most people. Which is another reason we love them. We all wish we could be like that. At least unless we are *really* beaten down, to the point were unseriousness seems threatening, heretical.