Rena

Rena is an identity secretly taken on by one of the Nakano quintuplets in disguise. She could be the mysterious girl that Fuutarou Uesugi encountered during his elementary school trip in Kyoto five years prior to the story. Rena can be referred to as "the Girl in the Photograph" that was taken with a young Futaro six years ago during the elementary trip to Kyoto.

Potential history

 * Related article: Young Nakano

5 years prior to the start of the story, Rena was seen defending the young Fuutarou Uesugi from getting into trouble with the police. Somewhere around that time, she would take a picture with him and Fuutarou would keep it in his student notebook.

Seven Goodbyes Arc
5 years later, Rena once again met up with Fuutarou Uesugi during his time of distress, wanting to provide him with some support. After exchanging her charm and taking away Fuutarou's image, she told Fuutarou that they will never meet each other for the third time and walks away before Fuutarou could reach her.

Kyoto Trip Arc
Rena then goes back on her promise during the Kyoto Trip arc, appearing next to Fuutarou after he talks to Yotsuba Nakano. Rena admits she used her mother's name, and wants him to guess her identity of the quintuplets. Tired and sick of the identity game, Fuutarou simply gets up and walks away, wishing to enjoy his school trip.

Identity
Ever since her introduction, various hints are given regarding Rena's identity.

Starting at Third Year Saga, Negi Haruba starts giving a more detailed hint or revelation about Rena.
 * 1) She has a charm similar to the charm Fuutarou bought five years ago.
 * 2) She gave Fuutarou her charm, which Fuutarou lost in the river, so one of the Nakano Quintuplets doesn't have the charm anymore (at the moment only Itsuki Nakano's one was shown before).
 * 3) Rena now keeps Fuutarou's photo of their younger selves. Itsuki dropped the same picture while she was putting away her items.
 * 4) Rena knows how Fuutarou used to look like when he was younger.