Funatsumaru.

I can't kill my favorite vampires! I've accidentally been reincarnated into the world of "Red Nosferatu," a romance otome game in which you fall in love with handsome vampires. And, of all people, it turns out to be Diana, the villainess "vampire hunter" who kills all the girls she likes! I'm going to die whether I kill the vampires or not! She must somehow avoid this death flag...!

In a world where humans and demons are in an endless war... One day, the demon king, Gil, arrives in the Kingdom of Hazelbard, a large human territory. Gil proposes a "truce" to the humans on the condition that neither side will invade the other's territory. However, due to a scheme by the queen, who is also his stepmother, to credit the agreement to her son Clemon, Princess Estelle is forced to marry the demon king in exchange... (Source: Cmoa, translated)

Eliana, the daughter of the Marquess, witnesses her fiancé, Prince Edward, and her half-sister, Phyllis, hugging each other! The shock reminds her of her past life, and she realizes that she is a villainess. In order to avoid her bad ending, she broke off her engagement and decided to become a magician. There she meets Zeon, a royal magician, and becomes his apprentice. However, Zeon has a secret identity that hides his identity even though he has the right to succeed to the throne, and pursues the enemy who killed his parents. Her former fiancé and her half-sister get in the way of the two who are gradually attracted to each other. An unexpected person awaits them after overcoming the ordeal...!