Because it is used as a nickname.
The nickname of a… girl!

The reason of the feminine is that the main character of the series is a woman who is (or used to be) a passe-miroir, i.e. who was able to pass through the looking glass, like Lewis Carroll’s Alice.

The name might also be a reference to Marcel Aymé’s passe-muraille, about a man who is able to pass through walls.

The head of the compound is actually passe: It’s a thing or person that passes through mirrors and in those verbs+objects compounds it’s always the verb that’s the head, in the same way the verb is the head of the sentence.

Of course, verbs don’t have a gender, and normally such compounds would get the less marked gender, masculine: un couvre-chef, un attrape-nigaud, un passe-partout, un perce-neige, un coupe-ongle, du lèche-vitrine, and so on.

However, when those V-N compounds refer to a human, they take the gender of their human referent. For a woman, thus, we’d say une casse-cou (a “break-neck”, a daredevil), une porte-parole, une lèche-cul, etc.

While I’m not familiar with the book series you speak of, it seems to be named after the occupation of its main character, a woman. From wikipedia:

Ophélie, l’héroïne de l’histoire. Elle est la fiancée puis la femme de Thorn, dont elle tombe amoureuse. C’est une liseuse, et une passe-miroir.

Nothing strange, then, about the gender of this term.