- Sarah Michelle Gellar as Bridget Kelly and Siobhan Martin
- Kristoffer Polaha as Henry Gallagher
- Ioan Gruffudd as Andrew Martin
- Nestor Carbonell as Agent Victor Machado
- Tara Summers as Gemma Gallagher
- Mike Colter as Malcolm Howard
- Zoey Deutch as Juliet Martin
- The pilot episode was filmed in New York City.
- Sarah Michelle Gellar began acting when she was only 4 after being discovered in a restaurant. Her first acting gig was a commercial for Burger King. She became a household name when she took on the starring role in Buffy.
- Geller spent years training in Tae Kwon Do and is psychically capable of doing many of her own stunts.
- Nestor Carbonell starred in the hit ABC drama series Lost as the never-aging island resident Richard Alpert.
- Before getting his role on Ringer Kristoffer Polaha starred in the CW's underrated drama Life Unexpected as Nate “Baze” Bazile.
Siobhan Martin: The identical twin sister who lives a life of wealth and privilege in New York (with a home in the Hamptons of course). She is in a loveless marriage and is/was having an affair with her best friend's husband. Shortly after reuniting with Bridget, she appears to have committed suicide by disappearing off her boat in the middle of the ocean.
Andrew Martin: Siobhan's husband. He is clearly unhappy in his marriage to Siobhan, and it appears as though she is the one who is single-handedly destroying the relationship.
Gemma Gallagher: Siobhan's best friend and decorator. She knows her husband is having an affair, but she has no clue that it happens to be with her best friend.
Henry Gallagher: Gemma's husband, unemployed writer... and the man who is having an affair with Siobhan.
Agent Victor Machado: The FBI agent in charge of protecting Bridget. Once Bridget escapes protective custody, he sets out on a search for Bridget, bringing him to New York where he meets "Siobhan."
Malcolm Howard: Bridget's AA sponsor and confidant. It's not clear, but it appears as though these two have an intimate connection. When Bridget takes over Siobhan's identity, Malcolm is the person she calls for guidance.
Juliet: Andrew's troubled teenage daughter who managed to get herself kicked out of another boarding school. Juliet and Siobhan don't get along, but she may have a shot with Bridget as her stepmother.