Onna Maou-sama wa Yuusha-kun wo Taosenai.

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)