Yoshinoya is the homeroom and art teacher for students like Yuno and Miyako at Yamabuki High School, instructing various art department classes. New students are often startled by her eccentric behavior. She possesses forest green hair styled in a hime cut and matching green eyes, standing 158 cm tall and noted as the most buxom character in her universe. She wears distinctive pink indoor slippers on school grounds that produce a recognizable squeak with each step.

Her most defining trait is creating and wearing elaborate, situation-specific costumes, though she avoids calling it cosplay. She designs and tailors all outfits herself, demonstrating advanced sewing skills. These creations range from seasonal themes to event-based attire and occasionally include risqué designs worn unabashedly before students. Combined with a tendency to make semi-ribald jokes during class or parent interactions, this habit frequently leads to reprimands from the principal, whom she actively avoids. Her outfits have caused misinterpretations, like when offering to repair student uniforms. Her tailoring skills are respected, evidenced by a commission to create a friend's wedding gown and bridesmaid dresses.

Despite her flamboyant, self-centered demeanor—where she calls herself a "princess" and openly praises her beauty and talents—she demonstrates genuine dedication to teaching. She exhibits perceptiveness and seriousness when advising students about futures or personal challenges, balancing eccentricity with compassion. She maintains a close friendship with school nurse Kuwahara, frequently seeking refuge in the infirmary when fatigued or evading the principal. Collaborations with fellow art teacher Ikezawa on projects are occasionally strained by thematic disagreements.

Artistically gifted beyond costume design, she contributed a campus statue that attained legendary status among students and faculty. The principal acknowledges her dual talents in art and education.

She lives with her parents as an unmarried, self-described "eternally seventeen"-year-old, explicitly stating no plans to marry. She has a younger brother, three years her junior, who is married with a son named Kento. She experiences severe motion sickness, as depicted during a bus trip. The surname "Yoshinoya" shares its name with a prominent Japanese beef bowl restaurant chain, which once led Miyako to misinterpret it as a bust size reference. A possible given name, "Kashiko," appears in a note within one episode, though unconfirmed.

Her portrayal remains consistent across the series' seasons, films, OVAs, and related media as an eccentric yet dedicated educator with flamboyant self-expression and deep, unconventional care for her students. No significant character development or changes in role, relationships, or behavior are documented.