Kouko Kaga (加賀 香子 Kaga Kōko) is the main female protagonist of the Golden Time series. She is a freshman law student who was formerly in love with Mitsuo Yanagisawa. She later becomes the girlfriend/fiance of Banri Tada.
Kouko has wavy waist-length light caramel brown hair that has little "flicks" (similar to curls) at the ends of her hair (her hair is always seen worn down which is almost never tied up, though she ties her hair up in a ponytail or bun when the situation calls for it), dark hazel eyes and large breasts. She wears a lot of stylish clothes in her daily life, almost always wearing high heels pink lip gloss/lipstick. Since Kouko 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.
Kouko's personality is one that changes depending on who she's with. Due to being raised in a wealthy household, she has formal and strict ways of expression that make her talk and act differently compared to those around her. She interacts with strangers with formality and kindness, but her true nature comes out more around love interests like Mitsuo Yanagisawa and later Banri Tada. She is extremely devoted to the object of her affection, pursuing them single-mindedly and being clingy to the level of stalking. While she is aware that this side of her seems to bother people - including the ones she likes - she can't change herself. She also has a harder time controlling her emotions around the person she loves. For example, when she discovers that Mitsuo transferred to a new college, she buys him a bouquet of red roses to congratulate him, only to lose her cool and hit him with the bouquet when she sees him. She also can't stand seeing other girls around the person she loves, becoming hostile and rude in those scenarios.
Despite being ditzy and happy on the outside, Kouko admits early on that she is truly very lonely at college. Most students believe she is too pretty and wealthy to befriend, so they ignore her. On top of that, her obsession with love kept her from reaching out to others on her own. When she does make friends, she often takes time to accept them. Kouko also suffers from low self-esteem, considering herself awkward and burdensome to those close to her. She also has an immature side to her that she rarely ever shows.
Kouko's relationship with Banri reveals more about her personality. Because he is willing to socialize with her early on, she quickly becomes relaxed around him. She becomes more open and starts to depend on him a great deal, even showing sides of herself Mitsuo doesn't see. Like with Mitsuo, Kouko dedicates most of her free time to Banri and works harder than necessary to impress him. She is constantly looking for ways to be a better girlfriend. She also has a habit of avoiding confrontation with Banri: even when he worries or wrongs her, she either blames herself or refuses to talk to him. All of this behavior is the result of her insecurities and low self-esteem. Despite being told otherwise repeatedly, Kouko thinks of herself as an unworthy lover and fears Banri will leave her. After learning about his retrograde amnesia and his past connection to Linda, her anxiousness deepens, though she tries to hide it. Because of all her doubts, she is slow to trust that Banri loves her and seeks to confirm his feelings. As he shares more of his past with her, she starts to help him face his problems and sort out his feelings. Overall, she is a good natured young woman who's easily hurt, but always tries to do her best for the person she loves.
Kouko 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, Kouko'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 in 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, Kouko is always being avoided by Mitsuo.
Kouko first met Banri outside the college entrance when she was about to slap Mitsuo Yanagisawa with a rose bouquet after the college entrance ceremony. Banri considered her attractive at the time, but she didn't feel anything for him. Kouko initially spent time with Banri in hopes of getting closer to Mitsuo, but she quickly becomes relaxed, kind, and honest with her feelings around him. In the seventh episode of Golden Time, they become an official couple. However, when Banri's original memories start to replace his current memories, she breaks up with him, unable to bear watching her boyfriend forget their relationship. However, she doesn't hesitate to take him back when he remembers her again.
Mitsuo is Kouko's childhood friend. She is obsessively in love with, but he rejects and avoids her. After Mitsuo confessed his attraction to another girl, she gives up on being with him, only to then fall for Banri Tada. Afterwards, the two remain friends, though they insult each other and argue all the time.
Kouko initially hated Chinami because she befriended Mitsuo, and in fits of jealousy she would insult and mock her. However, Kouko's behavior isn't because of Mitsuo: she is really only jealous of Chisami's knack for making friends, something she struggles to do. The trend of mocking her continues, but she eventually warms to her depite refusing to openly call her a friend. Regardless of her behavior, she has fun with Chisami, frequently helps her out, and is comfortable sharing personal problems with her.
- Kouko doesn't seem to know how to cook, as seen when she tricks Banri Tada into believing she was cooking yakisoba by herself when she actually poured boxed yakisoba. She is very embarrassed about this, even thinking about spending the summer vacation on cooking classes.
- Kouko is shown to have a reserved opinion towards sexual relationships when she accidentally sees Banri's erotic magazines and condom in a box. However, she later tries to entice Banri into having intercourse, albeit being a virgin and extremely nervous.