Zodiac is set in a world where people’s lives revolve around the Zodiac sign. Literally, these people live in the zodiac constellations. 16 year old Rhoma Grace is unlike her fellow Cancrians. Instead of reading the stars by mathematical equations and logic, she can’t solve for “x” to save her life, so it surprises her more than anyone when after the death of House Cancer’s guardian, she is chosen to take her place as “Holy Mother”.
Now, Rho sees something different in the stars, something dangerous, and something no one else can see. Dark Matter. Soon after, freak weather incidents start destroying the houses, including her own. Once she starts to alert her court, she starts getting attacked by someone through Psynergy. The guardian of a legendary “13th house” in the Zodiac system, Ophiuchus. The only problem is, no one believes her.
Across the Zodiac system, there are 12 different stories, 12 different myths about Ophiuchus. No one, except Rho’s best friend, Nishi, who risks her own public image to spread Rho’s message.
Frustrated that her own court won’t take her seriously, Rho decides to take matters into her own hands. She, along with her Guide, Mathias (who doesn’t believe her but is required to follow her everywhere) travels across the Zodiac system to the various houses, warning them of Ophiuchus. But, like Cancer, no one, with the exception of a strange boy from Libra, Hysan Dax, will take her her seriously.
Finally, after numerous attacks, deaths, and a lot of persuasion from a three-hundred-year-old guardian of Gemini, the Zodiac houses decide to send out an army to fight Ophiuchus. Unfortunately for them, Ophiuchus saw them coming. After killing Rho’s entire army (which included her Guard Mathias) and attempting to kill Rho herself, Ophiuchus escapes.Maddened with the army’s loss, the Zodiac council stips Rho of her title of Guardian of Cancer. Dealing with the loss of Mathias and her guardianship, Rho is determined to finish the task she started. Now, with Ophiuchus still at large, Rho must find a way to defeat him.
Rho starts out kind of shy, not wanting to be in the public eye. She doesn’t read the stars like she’s supposed to, using math and solving for x. Instead, she calls things as she sees them, noticing things that her teachers don’t. She thinks she’s going crazy, until Cancer names her Guardian for her abilities.
Rho’s court doesn't believe her when she says that she sees Dark Matter in the stars, which confuses her because she’s guardian for the very ability the court is doubting. When planets start getting destroyed, Rho decides that it is her duty to warn the other Houses about Ophiuchus.When the book ends, Rho is a brave character determined to protect the Zodiac, her Cancrian trait of caring for family shining through.
In the beginning, Mathias is a Lodestar, a Cancrian star reader and peacekeeper of the galaxy, who doesn’t talk much. Rho, having a huge crush on Mathias, asks him to be her Guide. Mathias doesn’t think he’s the right person for the job, but Rho insists.
As Rho claims to see strange things in the stars, Mathias doesn’t believe her, but continues to teach her how to be Guardian. When Rho runs off to warn the other Houses, Mathias is pretty clear that he’s only coming because it’s his job.
Over the course of the book, Mathias is surprised at Rho’s consistency and determination. He becomes kinder to her, and more protective of her. When Hysan Dax starts paying more attention to her, Mathias is not comfortable with it.
When Rho leads the army to defeat Ophiuchus, Mathias comes clean, telling her that he believes in her and that he cares about her. Kissing her goodbye, he sends her out in an escape pod to defeat the 13th house.Fortunately for Rho and unfortunately for Mathias, Ophiuchus blows up the entire army right after Rho escapes, including Mathias.
Hysan Dax is the most unpredictable character in this book. We are first introduced to him at Rho’s coronation, where he says that he’s representing House Libra. he disappears from the scene pretty quickly, leaving Rho very confused about the strange boy.
He reappears when Rho’s court takes Hysan’s spaceship without knowing he was still on it. When Rho explains what she’s doing, Hysan is one of the few people who believe her. He takes Rho and Mathias across the Zodiac system to warn each House.
When they reach the House of Lybra, Rho and Mathias discover that Hysan is the Guardian of his House. Mathias, who knows Hysan has been flirting with Rho, reminds her that a relationship between guardians is forbidden.
Before Rho can lead an army, she has to be approved by the Zodiac’s court. At first they don’t believe her, and Hysan is there to comfort her when all seems to be lost. Soon after that, the court approves Rho’s mission and gives her control of the army.When Rho goes to defeat Ophiuchus, Hysan’s role takes him to a different place than the rest of the army. He and Rho are the only survivors. By the end of the book, a relationship between the two isn’t clear, but is hinted at for later books.