Boku no Sekai wo Mamoru Hito (Volume)

is the first volume of the Boku no Sekai wo Mamoru Hito light novel series, written by Nagaru Tanigawa and Illustrated by Akifumi Orizawa. It was released under the Dengeki Bunko Imprint in November 2005.

Synopsis
"Did you finally come back?" As soon as he came home, the boy was greeted by a strange girl. "Isn't she late?" The girl lifted her eyebrows. Her name seems to be "Tsumon Ayaba Tsumugi". But how should I read it? Eventually she declared to the boy. "I will be your guardian from now on." The boys who had no reason to be guarded were just confused ... Since then, she says, "attackers" have come one after another! A mysterious magician holding an eerie stuffed animal to a little girl who self-destructs 5 meters away. Her < >, which was commonplace for boys, is breaking down !?

(source: Bookwalker JP - Translated)