Kōko is a highly attractive woman with large breasts by most boys. She has long wavy hair that goes just below her waist, with has little "flicks" similar to curls at the ends of her hair & hazel eyes. Her hair is always seen worn down which is almost never tied up, though she ties her hair up in a ponytail and a bun with or without braids. She also wears a lot of stylish clothes in her daily life with high heels and normally wears pink lip gloss/lip stick. Since Kōko comes from a very wealthy family, she can afford to dress in the latest fashion, which causes many girls at the university to be intimidated by her.
Kōko's personality is very hard to explain because it usually always changes when it comes to Mitsuo Yanagisawa and later Banri Tada. For example, when she sees Mitsuo transfer to a new college, she hits him with a bouquet of red roses. When she is with Banri, she is really kind and honest and depends on him a lot. Due to Kōko's wealthy and rich family, she is raised with formal and strict ways of expression. This often makes her talk and behave quite differently compared to the other students. However, when she is with Banri she feels relaxed and calm, and doesn't hesitate to share her inner feelings with him. Despite being ditzy yet happy on the outside, she often feels lonely and unseen. Koko honestly wants to be a good girlfriend to Banri, but she doesn't know how to and hopes that she can improve simply because Banri is her first boyfriend.
She is shown as a girl with devoted nature. Through out the series she pursues person of her affection single minded, which often gets to the point of stalking the person or being too clingy to them. She admits in front of Banri that she is aware that her this very nature seems to bother others, but she can't change herself. Kōko is also shown as a person with confidence when confessing her love and hold herself in high esteem for being a good girlfriend. However, she feels threatened / jealous whenever there is any girl around Mitsuo / Banri or someone takes care of Banri in her stead. And, she behaves rudely to them, such as her making fun of Chinami Oka. This feeling along with Banri's retro grade amnesia Kōko is always shown as an insecure lover. And time over time she tries to confirm feelings of Banri about her. However, as the time progress Kōko is convinced of Banri's feelings. After Banri shares his past with her she starts to help him face his problems and sort out his feelings. Overall she is shown as a person with a good nature, honest feelings, easy to be hurt and always tries to do her best for the person she loves. She is very pretty and nice to others.
Kōko is Mitsuo Yanagisawa's childhood friend and she has been with him for as long as she can remember. Both of them entered the same elementary school, high school and had once taken a calligraphy class together. Despite their closeness to one another, Kōko's love for him appears to be one-sided. She ends up following Mitsuo to his current college secretly as she knew that Mitsuo had been lying to her about enrolling to their affiliated college back when they graduated from high school. She holds back her rage for four months and finally slaps Mitsuo with the rose bouquet upon seeing him happily walking to the university with Banri Tada during the entrance ceremony. Due to her history with Mitsuo, primarily with her stalking habits and overly-obsessive love for him, Kōko is always being avoided by Mitsuo.
- She doesn't seem know how to cook, as seen in Episode 13, where she tricks Banri into believing she was cooking yakisoba by herself, until Banri saw her pouring boxed yakisoba. She is very embarrassed about this, even thinking spending the summer vacation on cooking classes.
- In Episode 7, Koko is shown to have a reserved opinion towards sexual relationships. She says to Banri to have things happen naturally (when she accidentally saw Banri's erotic magazines and condom in a box). However, in Episode 19, she tries to entice Banri into having intercourse, albeit being a virgin and extremely nervous.