Ingénue

The ingénue (pronounced /ˈænʒənuː/) is a stock character in literature, film, and a role type in the theatre; generally a girl or a young woman who is endearingly innocent and wholesome.
Typically, the ingenue is beautiful, gentle, sweet, virginal, and often naïve, in mental or emotional danger, or even physical danger, usually a target of The Cad; whom she may have mistaken for The Hero. Due to lack of independence, the ingenue usually lives with her father or a father figure (although in some rare cases she lives with a mother figure). The vamp is often a foil for the ingenue (or the damsel in distress, for that matter).

http://en.wikipedia.org/wiki/Ingenue_(stock_character)