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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 Nakano about Rena (Chapter 57)|
Rena has chest-length light-colored or dark-colored (dark red in the anime) hair that has bangs swept to the right side, light-colored or dark-colored eyes (dark blue in the anime), a well-endowed figure with large breasts and full pink lips.
In Chapter 57 Pg. 10-Pg. 12, Shimoda mentions that Rena "was such an incredibly beautiful woman." that her beauty has charmed every male and female student.
Rena is known to be a very serious, unsociable, strict and stern woman, especially when it comes to teaching her students as she would often punish her students with iron-fist sanctions, but she was also very forgiving and for that (as well as her appearance), she was adored by her students that there was a student fan club dedicated to worship her.
According to Itsuki Nakano's words, Rena was "a gentle and maternal female."
Rena was a single mother who raised five children. Before her remarriage with Maruo Nakano, she and the quintuplets were living in poverty like the Uesugi Family, where it is implied that the biological father of the quintuplets abandoned her and the children, which is why she advised the quintuplets to be careful when choosing men.
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 and her death was a shock to the quintuplets, who she had constantly reassured that she would be fine  and she is currently buried at the Nakano Family Grave.
Before Rena remarried Maruo Nakano, she was once married to this man who is the quintuplets' biological father. It is uncertain what exactly caused them to split up, but Itsuki Nakano states that he left after finding that Rena was pregnant with quintuplets. When she was alive, she advises the quintuplets to always be careful when choosing men , which shows Rena's regret for marrying 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.
Maruo is Rena's new husband. Before getting married, Marou was one of her students and often visited her in the hospital when she became ill.
- 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 run 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."