Manic Pixie Dream Girl -

The Manic Pixie Dream Girl has become a conventional character in films.

Film critic Nathan Rabin, who coined the term after seeing Kirsten Dunst in Elizabethtown (2005), describes the MPDG as:

“that bubbly, shallow cinematic creature that exists solely in the fevered imaginations of sensitive writer-directors to teach broodingly soulful young men to embrace life and its infinite mysteries and adventures.”

MPDGs are said to help their men without pursuing their own happiness, and such characters never grow up, thus their men never grow up

See more at TV Tropes: http://tvtropes.org/pmwiki/pmwiki.php/Main/ManicPixieDreamGirl