One day, a former noble young lady with nowhere to go meets a strange man who picks her up.

Kanon, the daughter of an aristocrat, got kidnapped by bandits for money, but her family abandons her and so they decide to sell her instead. Under those circumstances, she gets rescued from the bandits one day by a demon, the former demon king who will take her with him.

In the short span of time that she was rescued, Kanon suddenly got demon markings on her supposedly ordinary human body...

(Source: DLsite Girls Maniax)

Note: This is the original doujinshi that got serialized as an all-ages Shoujo manga in Kodansha's Palcy magazine: Announcement (Pixiv)