The Adventures of a Rune Master

Akimitsu Tsukasa was summoned to another world with his classmates, but due to the betrayal of the friend he thought was his best friend, he ended up wandering alone in an unknown world. Tsukasa drifts to Aiguille, where a horned race of demons live, living every day on the edge of death as a "freelance mercenary," relying solely on the out-of-date magic "In-Jutsu" (Seal Technique) he received as a gift at the time of the summoning. One day, Tsukasa's childhood friend, Matsuzaka Mihono, suddenly visited him, asking him to "save my sister, Shihono." What is the condition that he makes with Mihono?

(Source: GCN Bunko, translated)