TV-Series
Description
Chiyo Iin is the twelfth girlfriend of Rentarou Aijou and his younger, blood-related cousin. She is a first-year student at Ohananomitsu Middle School, where she serves as the president of Class 1-1. At the age of twelve, she is chronologically the youngest member of Rentarou's family.

Chiyo is a petite girl with a slim figure and hair that is typically depicted as brown or black, styled into two short pigtails while at school. At home or on days off, she usually lets her hair down. Her most distinctive feature is a pair of large, round-framed glasses, which are a cherished keepsake from her late mother, Chiri Iin. She maintains a neat and proper appearance, wearing her school uniform in the most conservative and regulation-abiding way possible, including a knee-length skirt and a done-up blazer.

Personality-wise, Chiyo is defined by her overwhelming seriousness, reliability, and strong sense of responsibility. She strives to be the textbook example of a class president and has a powerful, almost compulsive, dislike for anything she perceives as messy, disorderly, or against the rules. When confronted with such disorder, she has a distinct reaction, letting out a drawn-out groan often transcribed as Vuu- or Uuuhhh, and feels an urgent need to immediately correct the situation. This drive to maintain order stems from her home life, where, after her mother's death, she felt compelled to be the responsible one for her somewhat bumbling and unreliable father, Hiro Iin. As a result, she is exceptionally mature for her age and highly skilled in all forms of housework, particularly cooking. Despite her strict exterior, she secretly struggles with the pressure of the high expectations placed upon her and desires to be allowed to be vulnerable and imperfect like any other child.

Chiyo has a notable quirk related to her glasses. Because they originally belonged to her mother, they are slightly too large for her face and tend to slip off easily. If she loses her glasses, her entire composure shatters; she becomes helpless, anxious, and begins crying uncontrollably, reverting to a clingy, childlike state until her glasses are returned to her.

Her role in the story often serves as a comedic straight man, or tsukkomi, frequently reacting with horror and trying to rein in the more absurd or rule-breaking behavior of the other girlfriends. She was initially horrified by the idea of middle schoolers kissing and attempted to ban such activities, though she later rescinded this after realizing that a kiss can be a beautiful and happy expression of love.

Chiyo's motivation to be perfect is challenged and softened through her relationships. Her bond with Rentarou Aijou is central to her development. They first meet when Rentarou visits her home. After a series of comedic accidents engineered by her father, Chiyo finds herself hugging Rentarou to warm him up. At that moment, Rentarou tells her that she does not have to be so firm and proper because she is still a child, which are exactly the words she needed to hear. She confesses her love to him, and they begin their relationship as boyfriend and girlfriend, despite their initial awkwardness about being cousins. Within the family, she has a unique relationship with Kusuri Yakuzen. Chiyo finds that when Kusuri takes on her adult form and wears glasses, she strongly resembles Chiyo's late mother. This has led to a comforting, almost mother-daughter dynamic between them, where Kusuri can calm Chiyo down. She also receives guidance from Nadeshiko Yamato, who helps her become more relaxed and accepting of herself.

Over time, through her interactions with the other girlfriends and the encouragement of those around her, Chiyo begins to develop. She gradually loosens up, becoming less rigid and more adaptable. Her compulsive fits of groaning and crying occur less frequently, and she transitions into a calmer, more dependable little-sister figure within the group. She also becomes more open to intimacy with Rentarou and begins experimenting with her appearance, such as trying makeup and more fashionable looks.

Despite her small size, Chiyo possesses several notable abilities. She is a genius at homemaking, taking full responsibility for cooking, cleaning, and managing the household budget for her and her father. Her cooking skills are particularly praised, with her yakisoba being comparable to that of a classmate whose family specializes in the dish. She is also surprisingly strong, able to summon bursts of physical strength when her sense of order is triggered, such as fighting Rentarou to a standstill in a sumo match or knocking over several people with a single headbutt. Her sharp mind and diligence make her a natural leader in her class.