Kimi wa Mendou na Kon'yakusha

I thought I was the protagonist. I’m the CEO’s daughter. I’m the one engaged to him… Plus, I’m still a v-… Ah, sorry for rambling. Let me explain… She’s cheerful, honest, and friendly. He’s successful at his job, considerate, and handsome to boot. (Extremely handsome. Hehe.) Anyway, sometimes, the more challenging a relationship is, the easier it is burst up in flames… I’m his fiancee, but I’m more like the rival love interest who’s just in the way. Someone to create conflict and suspense. In other words, the antagonist they must defeat to get their happy ending.That’s right. She’s the heroine of this story. Not me.

Source: MangaPlaza