"Urenokori Seijo" to Yoba Retemashitaga, Mega Shinka shita Menhera Maou-sama ni wo Mochikaeri sa Remashita

In a remote land famous for its saints, today is an important day. It's the day when new saints are introduced to the powerful rulers of the other kingdoms. Most of the saints are young teenagers, except for Allie who is over 20 and is known as the "unwanted saint." However, when the most powerful demon lord appeared, he went to Allie right away, and... it's an unexpected reunion with someone she didn't recognize! It's a one-step romantic comedy between saint and demon lord!

(Source: Kindle Store, translated)