Mayu to Shoujo to Machi no Sakimori

is a Japanese light novel series written by Jiro Kurifu, with illustrations by Sakai Hamachi. The series began publication on November 10, 2008 by ASCII Media Works under their Dengeki Bunko publishing imprint.

Synopsis
There are boundaries in the city that are invisible to the human eye.

It's usually closed, but sometimes it opens its dark mouth in an unexpected place, inviting a lost person to "the other side." And even today, a high school girl suddenly disappears. A worried friend, Naoko Yamamura, consults with Michiru Sakurano. Michiru, who is a beautiful girl but rough, takes Naoko to a boy.

The boy's name is Mamoru Suwa. Descendants of those who have been watching the boundaries since ancient times-the guardians of the city. After hearing Naoko's story, Mamoru aims for "the other side" and begins to move.

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