Yotsuba nakano - The Quintessential Quintuplets: 10 Reasons Why Yotsuba Is The Best Quint

It's revealed that she met Futaro 5 years before the start of the story: after Futaro and the girl from the photo Yotsuba had spent the entire day together and both went to the inn the quintuplets were staying at, she was interested in him and played cards with him while Yotsuba was gone, with Futaro himself unaware that he was interacting with another quintuplet.

Through many trials and tribulations into finding out who the girl in the photograph was, it turned out to be none other than Yotsuba.