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No matter how scary or stone-faced she was, she was forgiven. She was loved and adored. That's because sensei was such an incredibly beautiful woman.
|― Shimoda to Itsuki about her mother, Chapter 57|
Rena was originally a single mother, having to raise five children by herself. Before her remarriage with Maruo Nakano, she and the quintuplets were living in poverty like the Uesugi Family. It is implied that the biological father of the quintuplets abandoned her and the children, which is why she advised Itsuki to choose men carefully.
She was a teacher at a high school where she was well-loved and respected by her students. Shimoda was once her student and was inspired by Rena to eventually work as a lecturer at a cram school.
She passed away five years prior to the start of the story on August 14 from an unknown terminal illness. Her death was sudden to the quintuplets, to whom she had been constantly reassuring she would be fine. She is buried at the Nakano Family Grave.
As a teacher, Rena was an incredibly beautiful woman that her beauty charmed every male students and female students. She has light-colored or dark-colored hair that is swept to the right and light-colored or dark-colored eyes.
Rena is known to be a very unsociable and serious person, especially when it comes to teaching her students, but she was also very forgiving and for that (as well as her appearance), she was adored by her students and there was a student fan club dedicated to worship her.
Before remarrying Maruo Nakano, Rena was once married to this man who is the quintuplets' biological father. It is uncertain what exactly caused them to split up, but it is hinted that he was a bad person. When she was alive, she advises the quintuplets to always be careful when choosing men. This shows Rena's regret for having married this man.
Itsuki was definitely close to Rena as Itsuki goes to her mother's grave on the 14th of every month. Rena also acted as a role model for Itsuki, who listens to her words, especially when it came to choosing men. Due to this, Itsuki aspires to become a teacher just as her mother once was.
- Nakano Quintuplets: “Looky! We gathered these up for you... Thanks for working hard again today... Thanks for everything you do... Huh...? She froze... It knew it, this wasn't...”
- Rena Nakano: “Seeing you five living healthily, to me, is the greatest happiness I could ask for.”
- — Rena Nakano's heartfelt response to the quintuplets' gift, Chapter 87
- The name of Toraiwa Hot Springs, the hot spring inn runs by her father, is a pun toward Rena's personality. Tora (虎?) literally means tiger, addressing the fierceness toward her students, and Iwa (岩?) translates to rock, which can be interpreted as "hard to read."