Onna Maou-sama wa Yuusha-kun wo Taosenai.

Synopsis

Abyss, the female demon king, is troubled by her inability to defeat the hero, the nemesis of the demon races. This hero is an inexperienced boy, still a long ways off. Defeating him should be easy!... and yet... The trouble is he doesn't know Abyss's true identity and gets attached to her as her adoring "disciple"...!? (Source: Akita Shoten, translated)