The Battlefield Saint: I Was Made to Marry a Duke Knight in Place of My Little Sister, but I’ve since Found Happiness

Unlike her sister, who is known as the saint, Fine is a simple girl who has been treated very unfairly by her family, a line of earls. She is sent to a dangerous battlefield to work as a military doctor when she meets the duke knight Leon. She saves him from dying with a healing spell, and later asks him to marry her! She suspects that he might be mistaking her for his sister, but...?