Ansatsu Hime: Ha no Kokuyouki wa Kouri no Ouji ni Tokasareru

Overview

She fell in love with the prince (target) she was supposed to assassinate A heretical royal romance Belrose has black hair, black eyes, and beautiful features. Her father gave her the mission of assassinating the handsome prince. At a ball at the royal castle, Prince Siegfried fell in love with her, but for some reason she ends up "pretending" to be Sieg's fiancee!? Furthermore, as they spend time together, the two gradually become attracted to each other!? What will become of this splendid and thrilling love affair with the prince...? (Source: Pixiv, translated)

Source: AniList