Ayumu Kasuga, universally known by the nickname "Osama," is a transfer student from Osaka, though born in Wakayama and raised in Kobe during her early years. Tomo Takino dubbed her "Osaka" anticipating boisterous Osakan stereotypes, but Ayumu defies expectations with her reserved, dreamy, and introspective demeanor. Her birth name remains scarcely recognized, even in formal class contexts, eclipsed entirely by her adopted moniker.

Standing 156 cm tall, she sports brown hair, dark brown eyes, and a pale complexion accentuated by unique highlights beneath each lower eyelid. Her attire consists of a pink long-sleeved uniform with a white collar, red ribbon, dark-brown miniskirt, white socks, and brown shoes. Known for drifting into abstract daydreams or zoning out mid-class, she surprises peers with sharp riddle-solving instincts and perceptiveness—like detecting Kaorin’s hidden feelings for Sakaki.

Academically grouped with Tomo and Kagura as a "Knucklehead" due to poor grades, she shares Chiyo Mihama’s physical ineptitude but paradoxically revels in typhoon sprints despite thunder phobia. Her bond with Chiyo centers on an obsession with the latter’s pigtails, which she imagines as sentient beings with magical powers. This fixation fuels a surreal escapade in "The Very Short Movie," where Chiyo’s detached pigtails trigger a chaotic classroom chase.

Typically serene, Ayumu’s rare irritations surface over imperfectly split chopsticks. Her humor leans macabre, envisioning cafes adorned with cat carcasses or crafting tales of murdering friends. Quirks include pollen allergies, aversion to spice and fizzy drinks, and a belief that flawlessly divided chopsticks invite fortune.

Localization adapts her Kansai dialect into a Brooklyn accent (English manga) or Southern drawl (anime dub), echoing her Osakan roots. Fan adoration for her eccentricity secured her seventh place in Newtype Magazine’s 2002 anime heroine rankings, the highest within her series.