Yuusha Party ni Kawaii Ko ga Ita no de, Kokuhaku Shitemita.

When Yoki was reincarnated into a magical world, he became neither a hero nor a demon lord, but instead became a middle-level subordinate demon guarding the castle. When the Hero's party attacked the demon lord's castle, Yoki was able to defeat them easily. But he fell in love at first sight with the beautiful cleric Cecelia!