Zia is a magician trained by the House of Life to fight gods, and she means serious business. Carter and Sadie first get a glimpse of her in London, when she and Desjardins show up too late to stop Julius from summoning gods. They run into her again in New York, where she grumpily saves their bacon from Set's minions.
Among her many talents (lighting things on fire, ticking off Sadie, making Carter feel uncomfortable), Zia is good at intimidating people. When Sadie gets a good chance to look at her, here's what she sees: "Her black hair was trimmed along her jawline and longer in the front so that it swept over her eyes. She had caramel-colored skin and pretty, vaguely Arabic features. Her eyes—lined in black kohl, Egyptian style—were a strange amber color that was either quite beautiful or a bit scary; I couldn't decide which" (11.33).
And the kids have good reason to be afraid of Zia: as a representative of the House of Life, and her orders are usually to kill or banish gods on sight. This includes godlings, and Sadie and Carter fall into that category. When Zia rescues them from Serqet in New York and brings them to the First Nome in Egypt, it's less of a yay-happy-feelings kind of rescue than a we-don't-want-you-dead-yet rescue.
In fact, when they first attempt to get a portal working in order to return to Egypt, Sadie innocently asks if they're going to use the portal to summon more gods. Zia's response: "Accuse me of that again, and I will cut out your tongue" (11.65). We're guessing she doesn't make a lot of friends with lines like that. Indeed, she tells Carter "I don't have time for friends" (17.52), which is usually code for "I have a hard time making friends" in teenage-speak.
Still, Iskandar regards Zia as his "best pupil, and she is wise" (15.119). So she's got that going for her. And when the plane she and Desjardins are on goes down, she uses magic to land it safely, in spite of the fact that it exhausts her resources to the point where she passes out.
Despite being super loyal to the House of Life, Zia has the courage to stand up to Desjardins in Las Cruces and tell him they need to work together instead of fighting amongst themselves. Now that takes some courage!
Zia's past is shrouded in mystery—even to her. Like Sadie and Carter, Zia comes from a family that has experienced tragedy. Her village was destroyed by a rogue artifact when she was eight, and she was the sole survivor, rescued by Iskandar and other magicians. However, even though Iskandar gave her pictures of her family, she can't seem to remember anything of her life with them.
Even weirder, there's a gap in Zia's memory right after she returns from the London to the First Nome. She remembers Iskandar taking her aside to say that she was in danger and that he would take her somewhere safe. But after that? She doesn't remember anything until he sends her to New York to find the kids, which must have happened immediately after that conversation, so… what's the deal?
Turns out that Iskandar really did stash Zia somewhere safe. He created a realistic shabti (the Egyptian magic version of a stunt double) and sent the shabti off to find the kids. This was the safest option for Zia, for a number of reasons: since hosting Nephthys technically makes her a godling, the magicians would have sniffed her out instantly. Plus, the magicians wouldn't be the only danger to Zia, as darker forces might want to get their hands on her to use or sacrifice her.
This is all well and good as a plot twist, but what does it mean for the budding flirtation between Zia and Carter, and the less-hateful-than-at-first interactions between Zia and Sadie? We don't know yet, but Carter swears to find the real Zia, wherever she is.