Ku Fei is a Chinese exchange student at Mahora Academy, recognized by her dark skin, blonde spiky twin tails, and green eyes, usually clad in her school uniform. Hailing from a mundane background in China, her limited Japanese skills cause academic difficulties and earn her the nickname "Baka Yellow" within the "Baka Rangers" study group. Despite this language barrier, she possesses exceptional martial arts mastery, specializing in Xingyi Quan and Bagua Zhang with additional knowledge of Baji Quan and Xinyi Liuhe Zhang. She stands as one of the "3A Martial Arts Elite Four" alongside Setsuna Sakurazaki, Kaede Nagase, and Mana Tatsumiya. However, she ranks as the weakest in unrestricted combat due to her lack of magic, relying entirely on physical prowess and ki control.

Her personality blends cheerfulness, impulsiveness, and a competitive drive. She thrives on training, seeks strong opponents, and savors nikuman from Chao Lingshen's restaurant, where she works part-time. She intensely dislikes Chao and Satomi Hakase's experimental inventions, frequently becoming their unwilling test subject. Early on, she allies with Negi Springfield, joining his "Baka Rangers" and later becoming his martial arts instructor after the Kyoto trip. Adopting the persona "Kuu Rōshi" (Master Kuu), she teaches Negi ki techniques and combat fundamentals, enabling him to pass Evangeline’s trials.

Ku Fei’s combat abilities show significant evolution. She dominated Mahora Academy’s martial arts tournament before the series began, inviting daily challenges. During the Mahora Festival, she defeated Mana Tatsumiya in a tournament match but sustained a broken arm, later healed. Her Pactio artifact, formed with Negi, is "Shinchintetsu Jizaikon" – a replica of Sun Wukong’s Ruyi Jingu Bang staff that alters its size and length at will. She employs advanced techniques like Shundō (high-speed movement), Koukikou (ki-based defense), and Shintoukei (internal damage strikes).

Her story involvement includes rescuing classmates during the Wilhelm incident and battling antagonists like Fate Averruncus. Though close friends with Chao Lingshen, she remained unaware of Chao’s time-travel plans until her departure. In the Magic World arc, she joined Negi’s "White Wing" group. After their gateway shattered, she survived as a freelance bodyguard, honing her skills and dismantling criminal organizations. Upon reuniting with Negi’s group, she demonstrated enhanced abilities, shattering stone pillars and challenging Negi to arm wrestling.