Karen Kujō is a main character in the story, known for being a half-British, half-Japanese girl with an inexhaustible supply of energy and enthusiasm. She has fair skin, hip-length pale blonde hair often styled with a small bun on the side, and light purple eyes. Her clothing frequently features Union Jack motifs, with her signature piece being a dark blue parka worn over her school uniform, reflecting her pride in her English heritage.

She possesses a carefree, happy-go-lucky, and charismatic personality that makes it easy for her to befriend nearly everyone in her class. This warmth often leads classmates to share food with her during breaks. She is also known for her playful and teasing nature, most famously directed at her friend Aya Komichi. After a minor misunderstanding when they first met, Karen gave Aya the nickname Ayaya, which she continues to use affectionately despite Aya's persistent protests. A distinctive verbal trait is her habit of speaking intentionally broken Japanese, frequently ending her sentences with desu yo, though she can speak fluently and lapse into fast English when shocked or upset. Karen has a remarkably fast metabolism, allowing her to eat very large quantities of food without ever gaining weight, and she shows a natural aptitude for sports even when she does not fully understand the rules.

Her primary motivation for coming to Japan was her deep bond with Alice Cartelet, her lifelong best friend from England. Karen has admired Alice ever since childhood, when Alice rescued her after she fell from a bridge and injured her ankle; she regards Alice as her hero. When Alice moved to Japan, Karen told her father she wanted to study Japanese culture, a fascination largely fueled by reading manga, and he allowed her to follow. Her family is considerably wealthy, owning a large English estate and an apartment block in Japan, though this wealth does not affect her down-to-earth and friendly interactions with others.

Within the group of friends, Karen often serves as a catalyst for energetic and comedic situations. Her role involves bridging cultural gaps between her English upbringing and her new life in Japan, and she shares strong bonds with each of the main characters. She enjoys playful interactions with Shinobu Ōmiya and has a close rapport with Yōko Inokuma, with whom she bonds over their shared struggles with poor academic grades and equally large appetites. Her relationship with her strict homeroom teacher, Akari Kuzehashi, starts with friction due to Karen's forgetfulness and habit of napping in class, but gradually Karen helps her teacher become more approachable. She also forms a close friendship with classmate Honoka Matsubara, who was initially fascinated by Karen's blonde hair.

Across the narrative, Karen shows noticeable development, particularly regarding her future. While she initially appears as an immature and carefree student who does not take her studies seriously, she matures when faced with the pressure of high school entrance exams. Alongside Yōko and Aya, she studies intensely, and her determination to remain with her friends culminates in her successfully passing the exam to attend the same institution as them. This demonstrates a deeper layer of loyalty and commitment beyond her usual lighthearted demeanor. Her notable abilities include her athletic prowess, her talent for making friends instantly, and her unique skill of understanding romantic subtext among her peers, being one of the first to notice Aya’s feelings for Yōko. She is central to the collaborative effort of putting together a play, where she and her friends reflect on their shared experiences while navigating the challenges of scriptwriting and costume design.