Hakyoku Yotei no Akujo no Hazu ga, Reitetsu Koushaku-sama ga Wakarete Kuremasen!

She plays the role of a greedy villain, makes Zain fall in love with her, and then dumps him! But the cold-hearted Duke unexpectedly becomes obsessed with her...

She's been reincarnated as Grace, a greedy villain who loves men, a minor character in a novel, and even a foil! If she doesn't toy with and dump the cold-hearted Duke Zain as the story demands, and create an opportunity for him to meet the heroine, war will break out and Grace will die. To avoid a bad ending, Grace tries to play the villain character, but her original earnest personality prevents her from fully transforming into a villain. Instead, the Duke, who was supposed to dump her and said, "I'll never break up with you," ends up doting on her...!?

(Source: Kadokawa, translated)