Letter of a Passing Girl (すれ違う少女の手紙) is a Unique Mission Type that follows the story of Meconopsis. There are a total of 7 stages, all of which are quite simple as the focus is on the story cutscenes. Each stage forces you to take the guest party along as your helper - which is always made up of Meconopsis and a few other Flower Knights.
A noble girl named Meconopsis living in the suburbs of Bergamot Valley faces land inheritance problems after the death of her parents. She has run away from her mansion with the hopes of at least being able to choose her own husband. She meets Physostegia by chance and asks her to act as a guide around the city as she collects information on the best husband.
After hearing the story of “The Knight Commander that sealed the Nidhogg”, Meko learns that she's the last of the family line of the Kodaibana Royal Family's personal merchants. Although surprised, she comes up with a crazy plan to both marry the rumored Knight Commander, as well as reviving both her own house and Kodaibana.