An ingénue is a young woman — especially in a play or a film — who seems innocent, sweet, and new to adult life. It is a French word English borrowed, and people often keep the accent: ingénue. It names a type of role, and sometimes the actress who is always cast in that kind of part. Though actually, it is not only “young and nice”: the point is that she has not had much experience of the world, especially love and danger, and the story often uses that. The word can sound a little old-fashioned now; people might just say the innocent or the naive young woman. It is not an everyday word on the street.