I'll Polish the Gloomy Count into a National Treasure

A popular novelist suddenly dies while planning the final chapter of her serialized novel. When she wakes up, she is inside her own story, reborn as a minor noble lady: the heroine's older sister.

In the original story, the handsome but somewhat gloomy count was supposed to end up with the heroine. But the heroine, saying she doesn't want a plain man, pushes the count onto her older sister instead.

Determined to steer the story toward its proper ending, she takes up the writing notes from her past life and sets out to give the count a makeover. But the count turns out to be a diamond in the rough who shines brighter the more he is polished!? As she gradually falls for him, and with her younger sister seemingly scheming behind the scenes, the story starts moving in an unpredictable direction.

(Source: Kadokawa, translated)