Kei Karuizawa is a teenage girl of average height with waist-length blonde hair, typically tied in a high ponytail, and striking purple eyes. Her appearance is well-maintained, reflecting her strong sense of fashion and self-care.

Her standard school uniform consists of a light blue dress shirt worn beneath a red blazer, with the sleeves rolled up, paired with a short white skirt. She also wears light blue calf-length socks and standard school shoes. A defining element of her uniform is the blue ribbon she consistently wears around her collar. In more casual settings, Kei modifies her uniform by tying her blazer around her waist and rolling her shirt sleeves up higher than usual, giving her a relaxed yet stylish appearance.

Kei is widely regarded as one of the most fashionable and stylish students in Class D, taking immense pride in her outward appearance. She carefully curates her look, maintaining a polished and trendy image that aligns with her social status among her classmates.

During sports events, Kei wears a white T-shirt, red sweatpants with a white stripe, and white trainers, adapting her style to match the athletic setting while still maintaining a neat and presentable look.

At the beginning of the series, Kei Karuizawa appears to be a carefree, spoiled, and self-centered girl who prioritizes her own enjoyment over academics or responsibilities. She spends much of her free time socializing with other girls in Class D and frequently uses her Private Points to buy clothes and accessories. Her bold personality and status as Yōsuke Hirata’s girlfriend a relationship she established just two weeks after entering the school quickly make her the de facto leader of the girls in Class D. She is both admired and feared by her classmates, as her influence allows her to dictate social dynamics within the class. Kei is also known for her audacious interactions with boys, remaining unfazed by any comments or criticisms regarding her flirtatious behavior.

However, beneath her confident and assertive exterior, Kei harbors deep emotional scars from her past. She is a highly self-aware individual who understands her own weaknesses and wears a pompous mask to conceal her vulnerabilities. Despite her seemingly careless and arrogant attitude, she is remarkably observant and intelligent, with a sharp ability to read people and analyze her surroundings. She often makes calculated decisions, ensuring that she remains in control of social situations. Her cynical outlook on relationships leads her to believe that her classmates do not genuinely like her, but rather tolerate her due to her status and influence within Class D. This insecurity makes her cautious in her interactions, carefully maintaining her image to stay in the good graces of her peers.

When dealing with people she trusts, Kei drops her façade and becomes straightforward and outspoken. This is most evident in her interactions with Kiyotaka Ayanokōji, as she frequently voices her frustrations with his lack of common sense in social situations. Unlike others, Kei does not hesitate to call out Kiyotaka’s flaws and actively encourages him to improve his habits, making her one of the few individuals who challenge him on a personal level.

Despite being in a highly competitive school, Kei initially shows little interest in academic success or advancing to a higher class. She prefers to enjoy her school life in a seemingly normal way, indulging in fashion and luxury, which results in impulsive spending habits. However, over time, she matures and begins saving her Private Points, ensuring that she has enough to protect herself from expulsion if necessary. While she dislikes studying due to inconsistent attendance during middle school, she is willing to put in extra effort when required. If her friends ask for help, or if Kiyotaka orders her to complete a task, she exceeds expectations and proves to be highly capable when motivated.

During her middle school years, Kei Karuizawa was known for her headstrong and competitive personality, which unintentionally led to conflicts with similarly strong-willed girls. As a result, she became the target of relentless and cruel bullying, suffering extreme physical and psychological torment. Her tormentors scribbled on her textbooks, stole her notes, and subjected her to public humiliation. They poured dirty water on her while she used the toilet, physically assaulted her, and filmed the abuse to spread among classmates for amusement. They placed thumbtacks in her shoes, left animal corpses on her desk, and once pulled down her skirt in front of the entire class. After swimming class, they would hide her underwear or even her entire uniform, leaving her defenseless and humiliated. She was also forced to confess to boys she had no interest in, pick up garbage with her mouth, and even lick the shoes of her tormentors.

Kei endured countless humiliations and severe physical abuse, leading to deep emotional scars and the destruction of her self-worth. The accumulated trauma resulted in her developing Post-Traumatic Stress Disorder (PTSD), which profoundly shaped her personality. Seeking salvation and protection, she entered Advanced Nurturing High School determined to rebuild herself. She adopted an arrogant and confident persona to make herself appear strong and untouchable, believing that acting tough was the only way to prevent herself from being bullied again.

As a result of her past experiences, Kei developed a survivalist mindset, prioritizing self-preservation above all else, regardless of the methods or sacrifices required. Her trauma-driven instincts became evident when Kiyotaka Ayanokōji confronted her about her past and attempted to manipulate her. Believing that power dictated survival, she offered up her body in exchange for protection, demonstrating the extent of her psychological scars. Kei revealed that this was not the first time she had been forced into submission, she had long accepted that no amount of crying, screaming, or resisting would change her fate. This past abuse caused her to develop a cold and detached emotional state, yet despite her suffering, she refused to surrender and resolved to keep fighting for survival.

To secure her place in the school, Kei gravitates toward powerful individuals, using their status as a shield to enhance her own security and influence. Recognizing her dependency on others, she views herself as a “parasite”—a weak individual who cannot survive alone. This calculated approach to self-preservation has allowed her to maintain her social standing within Class D, making her a trusted figure among the female students. Many naïve classmates seek her advice on relationships, believing she is experienced due to her confident demeanor. However, Kei privately admits that she has never even held hands with a boy before. Instead, she skillfully fabricates stories to maintain her image, reinforcing the illusion of experience and control over social interactions.

Kei Karuizawa is initially assessed as subpar by the Advanced Nurturing High School standards, primarily due to her frequent absences and below-average grades in middle school. Her poor academic record and lack of athletic accomplishments were key factors in her placement in Class D. Kei openly dislikes both academics and sports, but she has demonstrated that when she exerts effort, she is surprisingly capable of competing with others. Kiyotaka Ayanokōji noted that if she were to apply herself even for a short period, she could significantly improve her performance, much like Ken Sudō did with his Overall Ability Assessment (OAA).

Despite her seemingly weak academic and athletic abilities, Kei possesses immense mental fortitude and an indomitable will, as recognized by both Kiyotaka Ayanokōji and Kakeru Ryūen. She has endured countless hardships and traumas that would have broken most people, yet she remains resolute and refuses to succumb to despair. Her greatest strength lies in her loyalty, which she displays at crucial moments, even surprising Kiyotaka with her unwavering dedication. Her resilience in the face of adversity sets her apart, as she refuses to remain crestfallen despite facing overwhelming challenges. However, Kiyotaka believes that if he were to ever betray her trust, it could be the final breaking point for her, a sentiment Kei herself acknowledges.

Over time, Kei’s perceptive abilities have sharpened, especially after dating Kiyotaka for several months. She has developed an uncanny ability to detect subtle shifts in the atmosphere, expressions, and emotions of those around her. Most notably, she has managed to accurately predict Kiyotaka’s thought process, an achievement that astonishes him given his typically unreadable nature. In one instance, Kei’s casual speculation about Kiyotaka’s background was shockingly accurate, momentarily unnerving him. Their deep understanding of one another has reached a point where they can communicate through mere glances, interpreting unspoken words and emotions with ease. This is particularly significant, as even Suzune Horikita, who has known Kiyotaka longer, still struggles to decipher his true intentions.

The name Kei (恵) is written with the kanji “恵”, which means “favor” or “benefit.” This suggests a sense of grace or kindness, though it contrasts with her outwardly strong and sometimes manipulative personality. Her surname, Karuizawa (軽井沢), consists of the kanji “軽” (karu), meaning “light”, “井” (i), meaning “well” or “mine shaft”, and “沢” (sawa/zawa), meaning “marsh” or “swamp.” Additionally, Karuizawa is the name of a well-known resort town in Nagano Prefecture, Japan, recognized for its tranquil environment and affluent reputation. This could symbolize the sophisticated and composed image Kei projects in her social circles, despite the emotional struggles she conceals beneath the surface.