Junichirō Kagami is a 24-year-old man who serves as the central figure of his story. His background is marked by a sharp contrast between extraordinary potential and a chosen life of seclusion. As a teenager, he was a prodigy in the field of physics, gaining significant attention in the scientific community at the age of seventeen by publishing a controversial theoretical paper. This paper proposed a method for creating a teleportation device, inspired by a concept from a popular anime. Although the academic world ridiculed his idea, no one has ever been able to disprove his theory, though the technology required to build it would take centuries to develop.

Despite being recruited by leading research institutions around the world, Kagami declined all offers. He claims to suffer from a self-diagnosed condition he calls YD, which he describes as an inability to do anything he does not yearn to do. Consequently, he became a NEET, spending his days as a hardcore otaku, locked in his room. His primary activities include updating his anime blog, watching anime, collecting memorabilia, and passionately debating fictional characters, with a particular fondness for magical girl shows.

Kagami's personality is a complex mix of laziness, genius, and unorthodox principles. Initially, he appears selfish, childish, and thoroughly unmotivated by the real world, groaning at the prospect of any work that distracts him from his hobbies. He possesses a sharp, condescending wit and can be brutally honest, even mocking a student's dream of becoming a voice actress. However, his actions, however bizarre they seem, often reveal an underlying, if unconventional, sense of care. He takes deep personal offense to anyone disparaging anime or otaku culture, which becomes a serious matter for him. His teaching methods are less about conventional lecturing and more about using his extensive, seemingly useless otaku knowledge to create social experiments, develop complex video game challenges, or manipulate social situations for what he perceives as his students' own good.

His motivation for becoming a teacher is not intrinsic. He is forced into the role by his younger sister, Suzune, who is tired of his parasitic lifestyle and manages the family's finances, forbidding him from making purchases without her approval. Suzune secures him a temporary position as a physics teacher at his alma mater, East Shinmei High, and her stubbornness and violent threats with a baseball bat ultimately compel him to take the job. Over time, his motivation evolves from simple coercion to a more active, if still eccentric, engagement. He is later scouted by Koyomi Hiiragi, the chairman of the private Hiiragi Academy, who is impressed by his unusual teaching methods. At Hiiragi Academy, Kagami’s role expands from reforming stray students to directly challenging the principal's harsh methods, aiming to "remake" the school's oppressive system.

In the story, Kagami acts as a trickster mentor. He takes on students who are outcasts or who have lost their way, using his unorthodox lessons to help them confront their problems. His approach often involves creating elaborate, game-like scenarios that force students to rely on their own strengths, even if his role is that of a manipulative puppet master working behind the scenes. His key relationships are primarily with his students and his sister. He has a significant early impact on Minako Kanou, an aspiring voice actress who is being bullied, though his methods for helping her are highly controversial, involving psychological manipulation and threats of doxxing to scare her tormentors. He seems genuinely dedicated to his students' growth, but he remains oblivious to the romantic feelings some of them develop for him. His relationship with his sister Suzune is central; she is the only person he truly fears, yet their bond is strong, revealing a more vulnerable and caring side to him.

Throughout the series, Kagami shows a degree of development. He begins as a grating, self-absorbed buffoon who only cares about his own amusement, but as he invests more time in teaching, he learns more about himself and demonstrates that he can enrich his students' lives. He grows from a reluctant substitute to a teacher who actively seeks to challenge flawed educational systems, even at great personal risk. His notable abilities are vast, stemming from his genius-level intellect. He is not only a physics prodigy but also a gifted programmer, capable of building chat rooms with firewalls as powerful as those protecting the Pentagon and creating complex game programs designed to help students pass exams. His true genius lies in his ability to combine his academic knowledge with his deep understanding of otaku culture, using his analytical skills to read social hierarchies and devise clever, roundabout solutions to real-world problems. He is a classic example of a Guile Hero, winning his challenges through wit, bluffing, and audacious plans rather than brute force.