Okotta Seijo wa Hitori de Maou wo Taoshitai

The Trizal Kingdom: a land that has been tied to holy beings brought from other worlds since ancient times. As a new threat looms over Trizal, a new Saintess is summoned...a 17-year-old named Yuri. Appointed to aide her in her journey to become the kingdoms hero is a priest named Hamil, and Yuri is informed that she needs to marry someone from the royal family and defeat the demon king... but as she meets the prince he suddenly declares his opposition to their union?! Is that so?! I'll do it all myself then! With tempers high, Yuri sets out on a journey to save the world- her way! (Source: MangaPlaza)