Karou's only friend may not have any magical powers, but that doesn't stop Karou from describing the feisty, diminutive Zuzana as a fairy who is "likely to be rabid, and bite" (4.25). Sweet. Zuzana's a testament to the power of friendship, and how it's not an easy path to walk. It requires work to maintain.
See, Zuzana understandably gets upset at Karou's constant evasion of questions and her tendency to disappear for days at a time. And when Karou finally tells her the truth—the whole magical truth—we can't blame Zuzana for not believing her. Any normal person would think it was just more cooked-up lies, just more friend-evasion.
However, we're glad that Kishmish shows up as living proof that Karou is telling the truth. (Too bad the poor bird-thing burned up and turned into dying proof shortly thereafter. Ba dum tsh.) Karou needs Zuzana.
As an orphan, Karou longs for friends. And Zuzana is a good friend who complements Karou well. Like Karou, Zuzana is also artistic. She's into drawing and puppetry. She gives Karou someone to talk to about cute boys. She's funny, and she's loyal.
Basically, she's the perfect friend. Why do you think she hangs out with Karou, who is, we have to say, a little less than perfect when it comes to friendship? What makes their friendship so strong, and do you think it will last?