Watashi wa Succubus ja Arimasen

Liz, a young noblewoman, is a star pupil at her academy—talented, refined, generous, and the apple of everyone's eye. But the day a legendary hero comes to the academy to convalesce, Liz, in spite of herself, does the unthinkable... She asks for the hero's underpants! Liz, as it turns out, has a secret. She was a member of the hero's party until all her memories were stolen during a climactic battle—including the fact that she's a demon of desire known as a succubus! She can hardly believe it when she reawakens to her completely out-of-character powers, but unfortunately for Liz, she's about to find out that it's not so easy to defy your true nature...

(Source: J-Novel Club)