Kemuri to Mitsu

Himeko Hanatsuka, age 12, and Bunji Tsuchiya, age 30. Their relationship status is: "engaged." This story takes place during the Taisho Era, when Western and modern culture was beginning to spread in Japan. In this unsettling and vibrant atmosphere, the two, who call each other "Bunji-sama" and "Fiancée-dono" slowly develop their love, surpassing the 18 year age gap.

(Source: Kadokawa, translated)