Jo Jina, who is known by her Japanese name Yukari Tanizaki, is the homeroom and English teacher for the main group of students at a high school in Tokyo. She is a woman in her late twenties with long, wavy brown hair and a casual, carefree fashion sense. Her students address her informally as Yukari-sensei, a reflection of her unconventional and close relationship with the class.

Yukari is a highly dynamic character whose personality shifts dramatically depending on her mood. At her worst, she is selfish, irresponsible, miserly, and easily angered, often taking out her frustrations on those around her with little regard for proper conduct. She can be a tyrannical presence, mercilessly teasing the young prodigy Chiyo Mihama and occasionally resorting to physically hitting students with objects like rolled-up papers or chalk. Her behavior includes giving her class a study hall simply so she can abandon them to go swimming in the school pool. However, she also has a capacity for genuine kindness, capable of offering sweet and encouraging words, leaving supportive notes on students' tests, and sincerely believing in their potential. This duality makes her a source of chaos but also a character who is amusing and likable despite her flaws.

Her motivations are primarily driven by immediate gratification and a sense of jealousy. She is prone to fits of envy, such as when her friend and fellow teacher, Minamo Kurosawa, receives nicer birthday presents from the students. Her love for eating, especially Korean barbecue, is a powerful motivator, leading her to make extravagant wagers with her class. She also has a strong passion for video games and becomes genuinely upset when she cannot purchase a new release. Off the job, her primary motivation seems to be having fun, which she does by frequently dragging her friend Nyamo on expensive drinking parties, which Nyamo is often forced to pay for.

The most significant relationship in Yukari's life is with Minamo Kurosawa, known as Nyamo. The two have been friends since they were students at the same school. In the present, Yukari shamelessly exploits this friendship, expecting Nyamo to tolerate and enable her selfish, dysfunctional behavior. Despite this one-sided dynamic, their shared history and Nyamo's enduring patience suggest a deep, if strained, bond. Among the students, she shares a notable kinship with Tomo Takino. The two have very similar obnoxious personalities, being boastful, jealous, delusional, lazy, and selfish. Unlike others who are terrified of Yukari's behavior, Tomo embraces the mayhem, and she is the only one who can tolerate riding in Yukari's car. In contrast, Chiyo is often the primary target of Yukari's teasing and reckless behavior, which has a profound and scarring impact on the young girl's psyche.

Yukari possesses several notable abilities and quirks. As an English teacher, she is fluent in the language, though she speaks with a strong accent and sometimes uses her skill to claim intellectual superiority over Nyamo. She is also infamous for her terrifyingly reckless driving. Showing no concern for traffic rules, other vehicles, or the safety of her passengers, her driving has left deep mental scars on characters like Chiyo and Kaorin and has even caused the usually stoic Sakaki to lose her composure. Her parents' car, which she drives, is battered and known to the students as the "Yukarimobile". Yukari and Nyamo are both alumnae of the school where they now teach, a fact highlighted when Yukari appeared at a Sports Festival in her old school uniform. While she does not undergo a dramatic personality change, her character development is shown through the consistent balance of her two sides, a quality that forms a core part of the series' unique charm.