Itsuki Isozaki is a science teacher at Meisei High School, where he also serves as the homeroom teacher for the protagonist, Shoutarou Tatewaki. Described as a handsome biology teacher, his outward appearance is often contrasted with what those around him perceive as a disappointing personality. He generally projects a very laid-back and even uncaring demeanor in his daily interactions. This attitude is most starkly illustrated in his beliefs about personal autonomy; he has expressed the view that an individual has their own right to their life, which includes the right to end it, a stance that puts him in direct philosophical conflict with others.

Despite this casual and sometimes detached exterior, Isozaki is a well-respected educator whom students seem to enjoy and trust. His calm and even-keeled personality allows him to balance out the more eccentric and volatile nature of the brilliant osteologist Sakurako Kujou when they work together. His primary role in the story is as a supporting character, often acting as a point of contact for the main duo. However, he becomes a central figure in the series' final narrative arc, titled The Butterfly Vanished in November, which focuses heavily on his past.

Isozaki's motivations are deeply rooted in a past trauma involving his former students. Two years prior to the main events of the story, he was the homeroom teacher for three close friends: Futaba, Hitoe, and Minami. The mysterious disappearance of the student Futaba Nishizawa left a profound emotional scar on Isozaki, who harbors lingering guilt and a feeling that he could have done something to prevent it. This loss created a void in his life, which he attempted to fill by taking up the hobby of growing plants. When another of his former students, Hitoe Madoka, subsequently goes missing, he is driven to action, setting aside his typical passivity to seek out Sakurako's investigative help to find her and confront the unresolved pain from his past.

Throughout the series, Isozaki maintains key relationships with several main characters. He is the homeroom teacher for Shoutarou Tatewaki, which frequently brings him into the orbit of Shoutarou's adventures with Sakurako. He also has notable interactions with Yuriko Kougami, another student at the high school, with whom he engages in a significant debate over the ethics of intervening in a potential suicide, highlighting their differing worldviews. His most crucial relationship is with his missing former student, Futaba Nishizawa, whose disappearance serves as the emotional core for his character arc. In the climactic final mystery, he teams up directly with Sakurako and Shoutarou, providing his personal connection to the case as they work to uncover the truth behind the disappearances.

Isozaki shows subtle but significant development over the course of the narrative. He transitions from a character who preaches non-interference in the lives and deaths of others to one who actively engages in a search for a missing person from his past. His emotional trauma is brought to the surface, and by the conclusion of the final arc, he is shown consoling a found student and apologizing for his perceived past failure to protect her, indicating a re-engagement with his responsibilities as an educator. As a science teacher, his notable abilities lie in his academic knowledge, but in the context of the story, his primary function is not through any special investigative skill but rather through his emotional investment and his role as a connection to the victims in a case that is deeply personal to him.