Karen Kujō is a half-British, half-Japanese individual with fair skin, hip-length pale-blonde hair typically styled with a small bun on the left side, and light purple tsurime eyes reflecting her English heritage. Her clothing frequently features Union Jack motifs, such as a dark blue parka worn over her school uniform and red sneakers bearing the same design. Her family's considerable wealth is evident in their large English estate and ownership of a Japanese apartment block, though this does not shape her interactions.

She possesses a carefree, energetic, and charismatic personality, easily forming friendships where classmates often share food with her during breaks. Karen exhibits a playful teasing nature, particularly toward Aya Komichi, whom she nicknamed "Ayaya" after an initial misunderstanding; the nickname endures despite Aya's protests. She often uses intentionally broken Japanese in conversation, frequently ending sentences with "ですよ" (desu yo), though she occasionally surprises friends by demonstrating fluency. A fast metabolism allows her to eat large quantities without weight gain, and she shows aptitude in sports despite sometimes lacking knowledge of the rules.

Karen deeply values her friendships, especially the lifelong bond with Alice Cartelet from England. She greatly admires Alice, viewing her as a hero after being rescued following an accident where Karen fell from a bridge and sprained her ankle, solidifying their sister-like relationship. Her loyalty extends to friends like Shinobu Ōmiya, with whom she enjoys playful interactions, and Yōko Inokuma, bonding over shared traits like poor academics and large appetites. An unusual relationship develops with her strict homeroom teacher, Akari Kuzehashi; initial friction arises from Karen's forgetfulness and classroom napping, but she gradually helps Akari become more approachable.

Her background reveals she moved to Japan after telling her father about Alice's relocation, expressing a desire to study Japanese culture that he supported. This decision reflects her fascination with Japan, heavily influenced by manga which served as her primary language source before arrival. Adjusting to Japanese school life involved navigating cultural nuances and academic challenges.

Character development occurs across the series, particularly concerning her academic future. During the third year, depicted in "Thank You!!", Karen faces entrance exam pressure alongside Yōko and Aya, studying intensely. Her determination to remain with friends culminates in successful admission to their same institution, demonstrating growth beyond her carefree attitude. Post-graduation, she visits Alice and Shinobu in England, maintaining their friendship despite separation.

Her relationships evolve: Honoka Matsubara, initially a classmate fascinated by blonde hair, becomes a close friend after bonding over Karen's interest in working at Honoka's family restaurant. Interactions with Isami Ōmiya, Shinobu's older sister, involve admiration for Isami's modeling career, though they occur less frequently. Karen's role within the group often involves lightheartedness and bridging cultural gaps, though she occasionally makes perverted remarks or jokes uncommon among peers.

Inconsistencies appear, such as varying eye colors (purple or brown) across episodes and a tendency to lapse into fast-paced English when shocked or sullen. These traits contribute to her dynamic portrayal without diminishing her core loyalty and exuberance.