As Eve and Denny's daughter, Zoë pops into the world with a furry brother by her side, a brother who admits to helping deliver her even if all he really did was watch—that slacker. Enzo becomes Zoë's playmate, confidante, and honorary big brother. She melts Enzo's heart immediately, in a way that Eve never fully did, and he feels a certain responsibility for her, especially since Eve specifically asks him on the day she was born, "Will you promise to always protect her?" (6.20).
Wow, no pressure there.
Zoë also brings into the family a certain amount of warmth and magic that wasn't there before, and she represents for Enzo the best qualities of both Eve and Denny. She's smart, curious, a critical thinker, and when Eve passes away, she is able to handle the breakdown of her family dynamic with a strength and patient fortitude that she must get from Denny.
Seriously, at six years old, this kid shows she can handle losing a parent, being separated from another one, and kept away from him by her grandparents, through it all actively questioning why things are the way they are.
To put it in perspective, we were probably eating glue when we were six.