Symbolism, Imagery, Allegory
Zadia Bloodstone is one hot vampire warrior babe. She may only be a fictional character and a product of Nina's imagination, but she still does an excellent job realizing Nina's hopes and dreams—you know, while Nina lies around her house, practically debilitated by her vampiric infection.
For one thing, Zadia is totally gorgeous, while Nina is sickly and pale with a retro haircut (for more on Nina's looks, swing by her page in the "Character" section). Here's how Zadia looks: "a glamorous, slinky young girl with white skin, black hair, and ruby-red lips. She wore high-heeled boots and lots of black leather, as well as an ammunition belt. Her canine teeth were long and pointed, but she was stunningly beautiful nonetheless" (1.8). Yeah, Zadia definitely doesn't look sickly.
Nor does Zadia feel sickly—instead, she is competent and strong. When faced with threats, "She always lures them to their doom with her catlike grace and husky purr" (4.9), and she goes on rescue missions and fights villains. Nina? Not so much. Oh wait… actually, that's exactly what Nina does in the book. But when she does, it's for the first time—her life up until this has been boring and structured around feeling sick. Before Casimir is killed, the only real excitement Nina experiences is through Zadia.
We're not the only ones seeing Zadia as an escape from the doldrums for Nina, though—Nina may think Sanford is an annoying know-it-all, but he seems to nail it when he tells Nina that Zadia's " a symbol of your flight from reality" (1.67). By the end of the book, Nina comes around and decides to spend less time on escapist fantasy (like writing books about Zadia) and more time investing in her own life as it actually is. Seems like a healthy move to us, and if Zadia were real, we're thinking she'd agree.