Dante41 said: That's a Chekhov's Gun from the very first Manga chapter. She appeared with them in the background of a group shot, never been explained.
Shows up in Chapter 294+, especially 298.
Well, ... sort of. Actually appears to be her sister. However, going by the eye shape, the one in the group shot is Zazie herself.
From what I understand, it was going to be explained earlier, but he (or someone) decided the festival arc was running long so it got cut.