Motoansatsu Sha, Tensei Shite Kizoku No Reijo Ni Narimashita. (PASH! Books) [Light Novel]

[Machine Translation] An assassin who failed in his mission is reincarnated as Serena, a duchess. I used to be an assassin. She is frustrated by the discrepancy between her mother, who is a flower girl, and her sister-in-law, who deceives everyone around her in an attempt to kick her out of her life. She is isolated and shunned because of her cool-headedness, and finally decides to run away from home when she learns of her sister-in-law's engagement to the second prince. But then Evan, the first prince, shows up. The cold-blooded former assassin, who has never known love, moves forward in aristocratic society without fear!

任務に失敗した暗殺者が転生したのは、公爵令嬢のセレナ。「私は元暗殺者。なりすますなんて簡単だ」そう思っていたセレナだが、頭がお花畑な母親や、彼女を蹴落とそうと周りを欺く義妹との感覚のズレにいら立ち、生きづらさを感じていた。周囲からの嫌がらせを痛烈にかわしながらも、その冷徹さゆえに疎まれ孤立するセレナは、義妹と第二王子の婚約を知り、ついに家出を決意する。しかしそこに、第一王子のエヴァンが現れて―!?愛を知らない冷血な元暗殺者は、怯むことなく貴族社会を突き進む!