Yoshinoya is the homeroom and art teacher for Yuno and Miyako at Yamabuki High School, later teaching other students including Hiro, Sae, Nori, and Nazuna. She possesses forest green hair styled in a hime cut, green eyes, and distinctive pink indoor slippers that squeak. Living with her parents, she claims to be eternally seventeen and remains unmarried with no intention to marry. Her younger brother, three years her junior, is married and has a son named Kento.

Her defining habit involves frequently creating and wearing elaborate, self-made costumes tied to seasons, events, or situations; she avoids calling it cosplay. These showcase her tailoring skills. She sometimes offers to mend student uniforms, occasionally causing initial misunderstandings. Her artistic talent extends beyond costuming, evidenced by a campus statue she creates that becomes a school legend, acknowledged by the principal.

Despite her eccentric energy and occasional troublemaking, Yoshinoya deeply cares for her students' well-being and futures. During career counseling, she provides serious, perceptive guidance, sometimes worrying she is too harsh. Her mentorship significantly impacts Hiro, advising Hiro's pursuit of art education. She assists Hiro with intensive college exam preparation, leading to Hiro's admission to Ama Art University.

Socially, Yoshinoya exhibits shameless behavior, wearing revealing outfits near students and making semi-ribald jokes, often causing discomfort she seems unaware of. She displays egotistical tendencies, frequently calling herself a princess while praising her own beauty and skills. She notably fears the principal due to reprimands for unprofessional conduct and actively avoids his scolding. She occasionally expels fellow art teacher Mashiko from the art prep room while working on costumes. Her closest professional relationship is with school nurse Kuwahara; she seeks refuge in the infirmary when tired or hiding from the principal. She collaborates artistically with art teacher Ikezawa, though they often disagree on thematic elements.

Additional traits include suffering severe motion sickness. Sharing a name with Japan's largest beef bowl restaurant chain leads to jokes sometimes misinterpreted as references to her bust size. Her given name may be Kashiko, as suggested by a note in an episode.

Throughout the series, including the graduation arc focusing on Sae and Hiro, she maintains her supportive role. She appears during their final exams, university entrance tests, and graduation ceremonies, continuing her blend of eccentricity and mentorship as they transition beyond high school. Her presence contributes to the thematic exploration of change and continuity within the educational environment.