Gokujou Kishi Danchou no Yuruginai Dokusen Ai

Overview

"I made up my mind...that I would give my life and body to you." The Arnault Night Watch is an organization under the direct jurisdiction of King Arndt. Lieutenant Cecilia is second in command to Rudiger, the head of the Arnault Night Watch. He always playfully teases her, and Cecilia is in love with him. She has loved Rudiger since they were kids, but now their relationship is admiral and lieutenant. Cecilia keeps her personal feelings to herself, bottling them up inside... But one day, Cecilia hears about the rumor of women going to meet "Asmodeus" to become beautiful. Unexplainable, related incidents begin to occur, one after the other. Cecilia starts her investigation to protect the safety of the kingdom, but she soon finds herself teamed up with an unexpected someone who is also investigating the rumors...?! A mystery love fantasy with unwavering possessive love of a stellar knight commander! (Source: MangaPlaza)

Source: AniList