In the end, nothing changed even after I came to this school. No, that's wrong... Maybe I never had any intention of changing. For better or worse, it's always been the same for me. After all, I understand myself better than anyone else.
— Kei's monologue, Light Novel Volume 4
Kei is a teenage girl of average height with waist-length blonde hair that is tied in a ponytail and purple eyes. Her school uniform consists of a light blue dress shirt below the red school blazer with rolled-up sleeves and a short white skirt. She walks in standard school shoes with light blue calf-length socks. She predominantly wears a blue ribbon around her collar. In a more casual situation, she simply wraps her school blazer around her waist and rolls up her dress shirt sleeves above the usual norm. She takes immense pride and care in having a pulchritudinous outward appearance and is known in the class to be one of the most fashionable and modish student in her class.
At the start of the series, Kei comes across as a sloppy and spoiled girl who only cares about herself. She spends most of her time with the girls after school and uses her points to buy clothes and accessories. She is admired and feared throughout the class for her strong personality and her status as Yōsuke Hirata's girlfriend after only two weeks of entering the school, therefore establishing her position as the girl's leader of Class D. However, despite her looks and attitude she displays, she has her fair share of secrets that she cannot afford to reveal to others. She was unabashed and unashamed by the comments made by other people for describing her audacious moves on boys that caught her eye.
Underneath her pompous mask that she wears in front of people, Kei is a girl tormented by the ghosts of her past who can be weak and sensitive towards her surroundings. Even though she gives off the impression of someone who is careless, arrogant, and messy, Kei's true self is very logical and astute with a sharp mind. She has remarkable ability to read the environment and the people around her, and makes the correct decision often. She is a self-aware person who knows better than anyone her own character and limitations. She also seems to have developed a cynical view of her surroundings and relationships. She believes everyone hates her and only took an interest in her due to the position and status she has in the class, making her very wary about her actions towards her friends to remain in their good graces.
When interacting with someone she trusts, Kei is a straightforward and outspoken person. This is seen in her interactions with Kiyotaka Ayanokōji, as she has expressed many dissatisfactions with him during their time working together. She also often calls out his lack of common sense regarding ordinary matters such as maintaining conversations during the series, and is the only person thus far who is actively trying to improve his bad habits.
In a school that is anything but ordinary, Kei tries to live in the most ordinary way possible and spends most of her time having fun with her friends. She is very connected to fashion and fancy things, which contributes to her having an extravagant and lavish lifestyle causing her to spend money irresponsibly.  That said, she had gotten past this mindset and was shown to saved a great deal points just so she could save herself from expulsion if it ever came down to it. So far, she’s not shown much interest in academics or a great desire to reach a higher class. She often leaves to study for exams at the last minute and is not fond of studying in general. Her dislike for studying is mainly due to the lack of chances she had to attend her classes consistently during middle school due to her past. Kei entered the high school without much of the theoretical grade of middle school, which made her experience studying in high school not very pleasant. However, if her friends ever request her assistance or she was ordered by Kiyotaka to do something, she spares no expense and goes out of her way to achieve better result than expected of her.
As the series progresses, Kei has grown more considerate and fonder of her friends, as shown when she expressed her worries for Suzune Horikita during the sports festival. Even though she did not like her because of her high and mighty attitude, she could still empathize with Suzune due to her own experience of being targeted by bullies. This is displayed again when she helped Maya Satō in her love journey; even though her feelings were conflicting with her intention of helping her friend, she still did her best to help Maya succeed. She was also the only person to notice Kiyotaka’s birthday and sent him a happy birthday message, which surprised him and made him feel hesitant to delete it. She also felt touched by Maya’s desire to preserve their friendship even if they loved the same person, leading Kei to give her an affectionate hug. She had shown an immense amount of loyalty towards her classmates and loved ones, as when she decided not to betray them even if confronted by her trauma and threatened by the possibility of having her past spread throughout the whole school. Since Kiyotaka was also her first official boyfriend, she has also displayed an immense quantity of loyalty towards him. For instance, she glared menacingly at Sato for suggesting that she will take Kiyotaka for herself if they ever broke up and for insinuating that she will leach off from higher profiled men if she’s given the opportunity.
Through the series, Kei’s displayed strong determination and bravery, which allowed her to show her strength at the critical moment. From Kiyotaka’s description, Kei's strongest point is her indomitable will to carry out her goal to the very end, no matter the cost. She’s been often praised for her unfathomable mental fortitude, which only serves to strengthen her in the face of adversity, such as when she got cornered and bullied by Kakeru Ryūen and his group on the school’s rooftop. Despite facing an overwhelming opponent and having her mind destroyed after finding out Kiyotaka’s betrayal, Kei found the strength to fight again, not in Ryūen’s words or Kiyotaka’s promise, but in her desire to be proud of herself. She would even sacrifice her own well-being to protect Kiyotaka and give him the peaceful school life he always wanted. Kei’s self-sacrifice, doing something, not for herself but for someone else’s sake, is proof of her growth towards overcoming her trauma. This juxtaposes her previous twisted mindset and allowed her to finally stand tall and face her past. Thanks to that, she became one of the targets of Ryūen’s admiration and respect. Kiyotaka, who is another admirer of Kei’s resilience and willpower, had his expectations exceeded, and by his admission, underestimated the strength of her heart, promising not to repeat the mistake.  At the same time, Kei has reasserted her promise to protect and save him if the time comes.
Surprisingly enough, even after suffering direct and indirect abuse at the hands of many different people, Kei doesn't nurture any grudges against them. On the contrary, she was said to be somehow grateful to her middle school bullies because, thanks to other people not knowing about the bullying, she could restart again in this school and learn from the experience. Kei considers that her actions brought evil deeds to herself. After the rooftop incident, she is surprised with herself for forgiving Kiyotaka so easily and even jokes about it before getting lost in her delusions. In her own words, it would be impossible for an ordinary person to forgive Kiyotaka so easily, which makes her a very generous person. She’s also known to be a very intuitive person, being able to guess Kiyotaka’s background with unbelievable accuracy.
Kei can be very vain and full of herself at times regarding her appearance. She is confident that any piece of clothing or accessory will fit her because of her cute and attractive figure. If needed, Kei can even use her beauty attributes to flirt and lead another man to do what she wants, as shown with Machida and even Kiyotaka at a certain point. However, Kei also has an insecure side she hides from everyone due to the deep scar she carries below her left breast. From her view, a girl's body is supposed to be soft and fluffy and beautiful, and such an ugly scar would crush that illusion for the opposite sex. Ironically, because Kiyotaka didn’t show any disgust towards her scar, Kei started to see him differently from other men.
When in love, Kei exhibits some tsundere traits. She will try to deny her feelings at first, but she is not overly stubborn to the point of denying it forever. She is also not dense. When flirting, Kei tends to be playful and attempts to tease Kiyotaka all the time, although most of the time, she ends up being the one getting teased.  Kei believes a confession must be honest and show the person's true feelings. She also believes that a confession is a sign of courage and determination to convey a feeling and must be answered no matter what.  She loves to praise her boyfriend, but she also tries to keep his head down-to-earth, calling him out about his lack of common sense in some matters of their daily life. Kei’s displayed a considerable amount of jealousy towards her partner, but ultimately, she is shown to put her complete trust in him, even knowing he interacts with many different girls and accepted that it couldn’t be helped due to how cool her boyfriend is.
During her time in middle school, Kei was known to always be the headstrong and competitive type, unwittingly causing her to make enemies out of similar girls and becoming the target of severe and cruel bullying by their hands. They scribbled on her textbooks and stole her notes. They dumped dirty water on her while she used the toilet. They punched and kicked her and filmed it so they could spread it around the class for laughs. They put thumbtacks in her shoes and animal corpses on her desk. They even once pulled her skirt down in front of the class. After swimming class, they'd hide her underwear and sometimes even her entire uniform. They also made her confess to boys she doesn’t even like. They'd make her pick up garbage that was on the ground with her mouth and eat it. At times, she was even forced to lick shoes and those were considered the less disturbing experiences she had to go through.
Kei endured humiliation, after humiliation, and even worse accumulating experiences resulting in a very deep physical scar and the destruction of her sense of self and personality, which led to her developing Post-traumatic stress disorder (PTSD). Kei ran into this school searching for salvation and acted arrogantly to make herself look big and strong, trying to act tough so that she wouldn't be bullied again.
Due to her trauma, Kei developed a very twisted mindset based purely on the urge to protect herself from any kind of abuse or intimidation, no matter what kind of method she needed to use or the sacrifices she needed to make. The best example of this is when Ayanokouji Kiyotaka confronted her about her past and tried to blackmail her. She went as far as to offer up her body as per his request to protect her secret and place in the school. She revealed that it wasn't the first time someone had used force against her. She could cry, scream, or thrash around, but it didn't matter. She was everyone's victim, which caused her to become cold and robotic. But even though she had suffered so much, she was ready to fight again and wouldn’t give up without one.
In order to survive in this school, Kei tends to cling to someone who is valued highly and use that person's status and position to enhance her own. Due to this method, she has deemed herself as a parasite, a weak creature that can't survive on its own. Due to the fake persona she’s built up, many naive girls tends to seek her for advice on topics such as romance. However, she confessed that she has never held hands once with another guy but she was skilful enough to hoodwink them through her made up lies and fictitious stories.
Kei was assessed as being subpar by the school standard. Her frequent unexplained absences and below average grades in middle school were direct reasons for her being placed in Class D. She doesn't like academics and sports, but she is surprisingly able to compete with others in both when she is willing to exert effort. Kiyotaka deduced that if she worked hard for a short period of time, she would easily be able to attain significant improvements like Ken Sudō in her OAA.
According to both Kiyotaka Ayanokōji and Kakeru Ryūen, Kei has immense mental strength and an indomitable will to never succumb to the challenges of life. She has displayed the highest level of loyalty at the most crucial of moments when everything seems to be against her which even surprised Kiyotaka. Any normal person would had given up on life after experiencing just a fraction of what she’s been through, however, she has went against all odds and eventually found true happiness in the form of Kiyotaka. She is stalwart and will be herculean whenever Kiyotaka request her assistance, even astonishing herself sometimes at the extent she is willing to go for him. She has been shown to not remain crestfallen or depressed for long periods of time despite all the seemingly insurmountable difficulties she has faced. However, the last straw that will break the camel's back would be that if Kiyotaka ever betrays her, in his opinion, it may be her last and final breaking point, to which she also shares the same sentiment.
After dating Kiyotaka for a few months, Kei has picked up on the ability to sense the slightest bits of changes in the atmosphere, expressions and emotions of others. She has also time and time again correctly predicted the thought process of the stoic and enigmatic Kiyotaka which amazed him on how she can read him like an open book. For example, she frightened him after her supposed guess of his pedigree was almost directly on the dot. Their relationship is also close enough that just through brief glances at one another, they are able to understand the words conveyed in their eyes, be it positive or negative. This is especially notable since even to the observant eyes of Suzune, she is still unable to grasp his true intentions despite knowing him longer.
- (Monologue) God is cruel. My life was destroyed because of that nightmarish day. I lost my youth, my friends, and even myself. I have to fix that mistake. No matter how much people hate me, it'll be better than suffering like that again. The most important thing is protecting myself. 
- (To Kiyotaka) "Well, whatever. It's not like this is the first time someone's used force against me. After a while, I gave up trying to resist. That's right; I was everyone's victim. I became cold, robotic. I could cry, scream, or thrash around, but it didn't matter. So, why aren't you laughing? Why don't you laugh at the pitiful loser who's been bullied all her life?" 
- (To Kiyotaka) "I'm grateful to the people who bullied me, and the school that covered it up, No one here knows about my past. That's why I was able to become a new me."
- (To Kiyotaka) “It’s just... I don’t really like bringing up my past. I thought I’d never tell anyone about it. But I ended up telling you, and it felt so surprisingly fine that it’s kind of odd..."
- Ah is that so? I can’t understand since I no longer have anything to fear anymore.
- I thought you were really amazing yesterday. You got a bit of my respect, seeing you take the initiative for the class. But Kiyotaka is a different matter altogether. If you are that scared, your relationship with him will never begin
- Looking back right now, this was the first time Ayanokouji-kun and me spend time alone together. In my three years of school life, he would become the most important and irreplaceable person, but at this very moment I wasn’t aware of it.
- I know everything about myself, including my strengths and weaknesses. I know that none of the boys or girls like me. Even knowing that, I’ve never thought to change. But it doesn’t matter. It doesn’t hurt me anymore. Because for some reason, I want them to feel that way.
- As I went over the memory again and again, I realized something. “Wait. He touched me, didn’t he? He put his hands on me.” It hadn’t really sunk in back then, but that’d happened, hadn’t it? He touched my thighs. I’d never even held hands with a boy before. What in the world had he done to me?
- “I feel that, although I’m not very sure, but perhaps it’s a good thing that I’m able to communicate with you right now.”
- "Well, it can't be helped right? For someone with a communication disorder like you, there are only a few girls you could possibly make contact with".
- “It’s just interesting, when I think about you saying those words while worrying about your surroundings.”
- “Yup, as I thought you are the best… I’m never giving you up to another girl.”
- "That despite being betrayed, I didn't betray in turn and that it meant he would be safe. If I could help him gain the peace he desires, then that's not so bad."
- "No matter how many times you ask me.....I will 'NEVER', 'EVER'......tell you"
- Aaaaaaaaaaaaa, mou, aaaaaaaaaa! Inside my heart there was another me who was squealing while running around. There's no more mistaking it. I just have to admit it now. Seriously, seriously, seriously, my heart's seriously been stolen away. The arrow which had pierced my heart. The arrow of love which I could no longer take out. Is this even possible? Is it even fine for me to fall in love with someone who's been partly bullying me? But it's already too late. The power of this arrow is tremendous. I have, I have towards Kiyotaka-----really, seriously, I've fallen in love with him....
- This is fine. I’m fine with this. I repeatedly told myself that. I will break here though. But for some reason, I felt a little proud of myself. That despite being betrayed, I didn’t betray in turn and that it meant he would be safe. If I could help him gain the peace he desires, then that’s not so bad. That’s that then. Somehow, aren’t I cool? There never was anything interesting going on with my life but by cooperating with Kiyotaka, things got exciting and that’s not so bad. I had fun. How should I put it, almost like the heroine supporting her hero from the shadows? Although I didn’t understand a lot of what he did, it was still somehow strangely fun. Besides, no matter how it happened, it’s still a fact that I was saved. That’s why I regret nothing. I have no regrets. But, you know? The truth is, deep down inside, I still held out hope that he might come and save me. Those fleeting feelings—also exist, I guess.
- The name Kei consists of kanji that mean "favor, benefit" (恵) (kei).
- Kei's surname Karuizawa consists of kanjis that mean "light" (軽) (karu), "well, mine shaft, pit" (井) (i) and "marsh" (沢) (sawa/zawa).
- As D-Class is for students with defective traits, Kei's defect is her low grades and for frequent unexplained absences during middle school.
- Kei's ranking in the "Female Character" category for Takarajimasha's Kono Light Novel ga Sugoi! guidebook are as follows:
- For the 2nd Year arc, when asked about her aspirations for the next year, she answered 'To spend as much time as possible with the person I love. And... to become even just a bit more useful to him. That sorta thing?'
- You-Zitsu Light Novel — Volume 1
- You-Zitsu Light Novel — Volume 5
- You-Zitsu Light Novel — Volume 6
- You-Zitsu Light Novel — Volume 7
- You-Zitsu Light Novel — Volume 4.5
- You-Zitsu Light Novel — Volume 8
- You-Zitsu Light Novel — Volume 11.5
- You-Zitsu Light Novel — 2nd Year Volume 3
- You-Zitsu Light Novel — Volume 4
- Promotional Akihabara Posters
|Homeroom Teacher||Sae Chabashira|
|Class Representatives||Suzune Horikita • Yōsuke Hirata|
|Students||Kiyotaka Ayanokōji • Kikyō Kushida • Airi Sakura • Kei Karuizawa • Rokusuke Kōenji • Ken Sudō • Kanji Ike • Haruki Yamauchi • Teruhiko Yukimura • Akito Miyake • Haruka Hasebe • Mei-Yu Wang • Kokoro Inogashira • Maya Satō • Hideo Sotomura • Chiaki Matsushita • Satsuki Shinohara • Kayano Onodera • Nene Mori • Wataru Ijūin • Ryōtarō Hondō • Soshi Miyamoto • Kyōsuke Okitani|
|Homeroom Teacher||Sae Chabashira|
|Class Representatives||Suzune Horikita • Yōsuke Hirata|
|Students||Kiyotaka Ayanokōji • Kikyō Kushida • Airi Sakura • Kei Karuizawa • Rokusuke Kōenji • Ken Sudō • Kanji Ike • Teruhiko Yukimura • Akito Miyake • Haruka Hasebe • Mei-Yu Wang • Kokoro Inogashira • Maya Satō • Hideo Sotomura • Chiaki Matsushita • Satsuki Shinohara • Kayano Onodera • Nene Mori • Wataru Ijūin • Ryōtarō Hondō • Soshi Miyamoto • Kyōsuke Okitani|