the secret woman
Definition
An elusive or mysterious female figure, often used in literary or cinematic contexts to denote a character whose true identity, intentions, or past are deliberately obscured.
A colloquial reference to a mistress or a partner maintained outside of a primary, public relationship.
An archetypal trope in mystery and noir genres representing the 'femme fatale' or the unseen catalyst for a protagonist's downfall.
Examples
He spent years obsessing over the secret woman in the painting, only to realize it was just a portrait of his own reflection after a rough night out.
If your dating life is as dramatic as a Victorian novel, you probably have a secret woman lurking in the shadows of your LinkedIn profile.
Every neighborhood gossip relies on the existence of a secret woman to keep the local brunch conversation from descending into a discussion about tax returns.