Wait, how did I not know the Manic Pixie Dream Girl existed as a trope? Nathan Rabin gets credit for first calling her out:

[Elizabethtown’s Kirsten] Dunst embodies a character type I like to call The Manic Pixie Dream Girl (see Natalie Portman in Garden State for another prime example). The Manic Pixie Dream Girl 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. The Manic Pixie Dream Girl is an all-or-nothing-proposition. Audiences either want to marry her instantly (despite The Manic Pixie Dream Girl being, you know, a fictional character) or they want to commit grievous bodily harm against them and their immediate family.

The Onion A.V. Club lists sixteen examples and further clarifies just what’s wrong with this archetype:

Like the Magical Negro, the Manic Pixie Dream Girl archetype is largely defined by secondary status and lack of an inner life. She’s on hand to lift a gloomy male protagonist out of the doldrums, not to pursue her own happiness. In the late ’60s and early ’70s, MPDGs often took the comely form of spacey hippie chicks burdened with getting grim establishment types to kick back and smell the flowers.

She’s simply awful. She’s the navigable air duct of female antagonists, something that exists only for cinematic convenience. Let’s stop using her.

Like villains, love interests need to have a plausible reason for why they’re there and what they want. Always ask yourself, “What would this character be doing if the hero never showed up?”

If you can’t answer — or if the answer is boring — you need to go back to the drawing board.

There’s nothing wrong with kooky females, by the way. Anna Faris has made a career of them. But in films like The House Bunny, it’s always clear what she’s after — and it’s never about getting a nice guy to loosen up.