John claims he does not have a specific "type" of women he is attracted to (see for example in RW136 and RW143), but he sometimes calls those women for Manic Pixie Dream Girls and for example used the term on his High School girlfriend Kelly Keefer (see segment in Friends from RL252)
The phrase itself has been coined in 2007 by film critic Nathan Rabin in this article about the movie Elizabethtown as a movie trope that in this example was embodied by Kirsten Dunst's character (see also Wikipedia entry):
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.
Another explanation can be found here:
The Manic Pixie Dream Girl is here to give new meaning to the male hero's life! She's stunningly attractive, energetic, high on life, full of wacky quirks and idiosyncrasies (generally including childlike playfulness), often with a touch of wild hair dye. She's inexplicably obsessed with our stuffed-shirt hero, on whom she will focus her kuh-razy antics until he learns to live freely and love madly.
The term does not seem to refer to the appearance of a woman, but merely to her character, although it has been noted that many Manic Pixie Dream Girls have been played by Jewish actresses. Merlin often jokingly refers to John's type of girlfriends as a "small sturdy Jew in combat boots" (RL12, RL240, RL274), or "Jewish girls in combat boots with a cotton dress and a big shock of tangled curly Lollobrigida hair and lipstick that was too red" (RL307, RL323).
