Set 400 years in the future, after a nuclear blast pretty much destroyed everything. Now, humans have survived but babies are always born as boy/girl twin.  One twin is “perfect”, the other has some kind of mutation.  The abnormal twin is branded and labeled an Omega, and sent off to live in  Omega-only villages.  Omegas are second-class citizens at best, but the Alphas need them–when one twin dies or is seriously hurt, the other one is too.  Omegas are forced to live in shabby villages, on poor land, and are required to pay unreasonable tithes. In desperation, some of them  move to “refuges” where they are given shelter in return for labor, but they will never really be able to leave.  Alphas and Omegas both fear any kind of technology from the Before, to the point that even touching ruins is taboo.

Cass:  our progagonist.  Cass is a seer (which makes her an Omega), and has visions of the future, past and present.  She can also “feel” places, and navigate places she’s never been.  In the first book, she is put in a “Keeping Room” for years at the order of her powerful twin, Zach.  Keeping her locked up will keep him safe. Cass escapes and resuces Kip, then the two of them find their way to the Omega resistance.

Zach: Cass’s Alpha twin.  He has become a powerful leader in the Council and is known as the Reformer.  In spite of the taboo, Zach has created machines called tanks, which will hold people in suspended animation indefinitely.  He, and the General (another Council leader) see this as the way to solve the Omega problem.

Kip:  During her escape, Cass first comes across the tanks, and rescues a man from them.  He has no memory of his previous life; Cass names him Kip.  They travel together and fall in love.  At the end of book 1, we learn that Kip’s twin is the Confessor, an evil Omaga seer who has aligned herself with Zach and the Council. Kip kills himself by jumping off a high platform, killing the Confessor at the same time.  In book 2, Cass learns that Zack took Kip’s and the  Confessor’s bodies, somehow brought them back to life (but barely) and tanked them in the Ark.  Knowing Kip would never want this, Cass pulls Kip from the tank, letting him rest in peace at last.

Piper:  leader of the resistance.  On the island, he refuses to have Cass killed, even though it would kill Zach at the same time.  When the Alphas attack the island, many Omegas blame him and Cass for all the death and destruction.

Zoe:  Piper’s Alpha twin.  She has chosen to remain with her brother, and fight for the resistance, in spite of being an Alpha.  In book 2, she disappears while she, Piper and Cass are searching for the Ark.  She reappears at the end of the book, having located the presumed lost ship, the Rosalind, which has managed to make contact with some people from “Elsewhere.”

The Ringmaster: another Council leader.  He joins the resistance to free the town of New Hobart, because he believes in the taboo and thinks Zach and the General are leading the people towards disaster by using Before technology.  It is not exactly an  easy alliance between him and the resistance.

Elsa:  a woman of New Hobart.  She took in Cass and Kip when they were on the run and then helped them escape the town when the Council showed up to wall it in.  Cass finds her again in book 2, after New Hobart has been freed, but not before the children of the town have been tanked, and later killed, by the Council army.  Elsa’s husband Joe had found papers that described the Ark.

The Ark:  an underground bunker built to survive the blast.  Papers found indicate that the scientists of the Ark had discovered a way to stop the twinning, but the method is not described.  It is implied that eliminating the twin phenomenon will not eliminate mutations. There are also indications that they had contact with Elsewhere.  In book 2, Cass and Piper find the Ark (as does Zach), and discover that its residents had tanked themselves, in hopes of being awakened.  This is where Zach found out about the tanking technology; he and the Council have also taken the technology that caused the original blast, and have plans to use it on Elsewhere.  The Council does not want the resistance to find the people of Elsewhere and learn how they stopped the twinning.

Paloma:  a woman from Elsewhere.  The crew of the Rosalind found her and she came back with them, as something of an ambassador.