Iin Chiyo (伊院 知与 Iin Chiyo) is Rentarou's 12th girlfriend in The 100 Girlfriends Who Really, Really, Really, Really, Really Love You by Nozawa Yukiko and Rikito Nakamura. She is also Rentarou's cousin.
Chiyo is petite with short hair tied into small twintails, along with glasses.
Chiyo is reliable and tidy which led to her becoming class president. But, she secretly dislikes the high expectations others place on her.
When she takes her glasses off, she becomes anxious and begins crying uncontrollably.
She met Rentarou when he came to visit her and his dad (Rentarou's Uncle), and due to having a high position of responsability to everyone else, she acted quite firm and tidy during Rentarou's visit. However, Rentarou calms her by telling she doesn't have to be firm all the time, thanks to this, she confesses to Rentarou that she likes him.
Her father (and only parent) Hiro, Rentarou's uncle, approves of her relationship with him after a young Rentarou once pushed him away from a speeding truck, believing Rentarou to be the best boy in the world as a result.
While Rentarou had his reservations against dating his cousin, despite knowing the fate she would have if he did not accept her, after witnessing her honest, pure confession, Rentarou accepts her feelings towards him, thus making her his 12th girlfriend.
Some time later, Rentarou introduces her to the rest of the girlfriends, thus, Chiyo becomes a member of the Rentarou Family.
- Iin (伊院) is made of 伊 (Italy, but may also refer to 伊呂波, meaning "fundamentals") and 院 (institution, or school).
- Chiyo (知与) is made of 知 (wisdom) and 与 (to participate, or award).