Your Highness, She's Here to Protect You, Not for You to Fall in Love with!

Elda Ragosh-an elite female knight-is determined to serve Princess Regina with her own life. When she goes on an undercover mission as a lady, the Princess's handsome, yet shy brother Darius falls in love with her. Her new mission: turn down the prince and set him up with someone else!

(Source: MangaPlaza)