One of the most chilling examples of this is Hedra in the film Single White Female.
From watching videos and studying the subject, I believe these characters often enter our lives, when we are in a weakened state. In this clip of Single White Female, Hedra ( the toxic flatmate) shows up, just as Alison is crying over a broken relationship. I watched this film when it first came out but have only begun to really understand it now, after my recent experiences. Hedra over time transforms herself into Alison, even eventually adopting Alison’s haircut and red hair.
Sources:From Surviving To Thriving From Surviving To Thriving