Throughout the book, we get many flashbacks. Each of the three women gets a section in the book in which they have a specific flashback about their childhood/their relationship with their parents. They also get a flashback in which they discuss in length how Zenia came into their lives.
Based on the flashbacks the reader gets about Tony, Charis, and Roz's childhoods, it becomes very clear that Zenia uses what she knows about their pasts in order to manipulate them.
For example, Zenia knows about Tony's mother leaving her and her father's suicide and at first uses these facts to antagonize her. She spins a story about how her own mother made her sell her body during the war. She specifically mentions war because she is aware of Tony's fascination with it, but she also makes out like Tony's hatred of her mother is not as justified as her own.
With Charis, Zenia knows that she is a deeply spiritual person. She comes to her in a place of illness and neediness, something that Charis can relate to. Zenia is able to appear weak in order to weasel her way into Charis' life. At first, she is kind and complacent. When she gets Charis alone, however, she says things Charis does not want to hear and instead becomes this spiteful, angry person which upsets the aura Charis feels.
Zenia first appears as a waitress when Roz and Mitch are out to lunch. Roz asks her what she's doing there and Zenia tells her that she's doing undercover research on sexual assault. Zenia presents herself as confident and prideful, personality traits in a woman that Roz looks up to and wants for herself. She seeks out Zenia because she craves that deeper friendship, only she gets more than she bargained for.
As far as attracting and controlling the men in their lives, Zenia manipulates them in a similar way as she does the women. She presents herself in a way that she knows they like. They find vulnerable, thus desirable. Just as a quick example, Zenia manipulates West and Tony's love for West in order to get her way. When it wasn't enough to just take Tony's money, Zenia had to come and take West from her not once, but twice because she knows that's how she can hurt the three women the most: by taking their love.