This is a term used to refer to a young girl that Fuutarou Uesugi encountered during the Kyoto school trip five years prior to the main story. She has made several appearance in Fuutarou's numerous recollections in the series. She can also be referred to as the Girl in the Photograph.
She is one of the Nakano's quintuplets. However, due to the quintuplets identical appearance at the time, it can be said that Fuutarou met with at least one of the quintuplets.