If this novel were a film noir and if sixteen year old Alex was a few years older, she would be described the same way as such women in such films are - as a "femme fatale". This is a woman (or in Alex's case, a young woman) who knows she's in trouble or attractive, often both, and uses a hapless, well-meaning man to achieve her often murderous ends. Calculating and self-centered, the femme fatale is a user, a liar, and a temptress, someone with drive and determination but whose intentions and actions are defined more by selfishness than compassion or affection.
