Karen Kujō is a half-British, half-Japanese girl from a wealthy English family, her Japanese father and British mother giving her distinct blonde hair and predominantly English features. She grew up with Alice Cartelet, forming a deeply loyal, sister-like bond. Learning of Alice's plan to study in Japan, Karen convinced her father to let her move there by expressing a desire to immerse herself in Japanese culture, enrolling at the same high school as Alice and Shinobu Ōmiya.

She possesses long, pale blonde hair, typically worn loose with a small bun on the left side of her head, and light purple, tsurime-shaped (upturned) eyes, though their color appears inconsistently brown in some anime scenes. Karen frequently incorporates British elements into her attire, such as a dark blue Union Jack school top, red Union Jack sneakers, and a Union Jack parka worn over her uniform. Her casual clothing blends playful and feminine styles.

Energetic, carefree, and excitable, Karen bubbles with hyperactivity and a fearless approach to new experiences. She forms friendships effortlessly, becoming popular enough that classmates willingly share food with her. Despite occasional immaturity or selfishness, she values loyalty and affectionately teases close friends like Alice and Aya Komichi, whom she nicknamed "Ayaya" after a misunderstanding. Karen is one of the few in her friend group who makes perverted comments. She deliberately speaks broken Japanese peppered with English phrases, surprising others when she occasionally switches to fluent Japanese learned entirely through manga. When shocked or upset, she reverts to rapid English speech. She exhibits a fast metabolism, consuming large quantities without weight gain, and excels at sports, performing well even without knowing the rules.

Her family's significant wealth, known to Alice but surprising others, is shown through her grand, house-like motorhome, ownership of an entire apartment block in Japan, and flashbacks of her expansive, lavishly decorated English home. Karen frequently expresses admiration for Shinobu with declarations of "really really really loving" her, sometimes inciting jealousy in Alice. She bonds with Yōko Inokuma over poor academics and shared enthusiasm for eating. Initially clashing with her second-year homeroom teacher, Akari Kuzehashi, due to forgetfulness and classroom antics like sleeping, Karen gradually helps Akari overcome her stern reputation by persistently engaging with her, revealing Akari's underlying kindness and their shared tsurime eyes.

Throughout the series, Karen deepens friendships, such as with Honoka Matsubara, who admires her blonde hair. Honoka overcame shyness to befriend Karen, exchanging contact information, spending time together at Honoka's family restaurant, and receiving encouragement from Karen before a tennis match. Karen accompanied Alice on a trip back to England to visit their parents, temporarily leaving Shinobu. After high school, she attended the same college as Aya and Yōko. In later manga installments, she remains active in the friend group, participating in everyday activities without significant changes to her core traits.

Her name means "pretty" in Japanese, her flower motif is the Anemone, and her character serves as a homage to Jotaro Kujo from *JoJo's Bizarre Adventure* due to shared surname, mixed heritage, and British maternal lineage. She appears in every anime episode except Episode 2 of the first season.