Gaku Yashiro is the primary antagonist of the story, introduced as a well-respected and charismatic fifth-grade teacher at Mikoto Elementary School. He is a man of average height with black hair and black eyes, often seen wearing glasses and a trench coat. His professional and approachable demeanor allows him to easily connect with his students, and the protagonist, Satoru Fujinuma, initially looks up to him as an ideal adult and a father figure.

Beneath this affable exterior, however, lies a calculating and remorseless serial killer who preys on children. Yashiro is a classic sociopath, possessing high intelligence and a charming personality that masks a complete lack of empathy and an insatiable need to fill a void within himself. His motivations stem from a deeply rooted sense of emptiness, a condition he first recognized in a childhood incident involving a box of hamsters. After drowning the litter, he became fascinated by the sole survivor that managed to stay alive by climbing onto the bodies of its siblings, naming the hamster Spice. This event became a defining metaphor for his worldview, connecting to the classic Japanese short story The Spider's Thread, where a sinner in hell is offered a thin thread as a chance for redemption. Yashiro came to see a "spider's thread" above certain individuals, and he believed that by severing this thread through murder, he could experience a fleeting sense of fulfillment or "spice" in his otherwise hollow existence.

His role in the story is that of a hidden predator who uses his position of authority to identify, isolate, and murder vulnerable children, often framing others for his crimes. He is directly responsible for the original abduction and murder of Kayo Hinazuki, one of his own students, and later kills Satoru's mother, Sachiko, to protect his secret in the original timeline. When Satoru travels back in time to prevent these murders, Yashiro becomes his archenemy and primary obstacle. His key relationship is with Satoru, which evolves into a complex and twisted obsession. After Satoru, using knowledge of the future, exposes him and nearly dies, Yashiro is so intrigued by Satoru's unique ability that he waits for fifteen years during the protagonist's coma, tending to him in the hospital just to learn his secrets. This obsession becomes Yashiro's new "spice," replacing his urge to kill.

Throughout the narrative, Yashiro exhibits a notable lack of development in terms of redemption. He shows no genuine remorse for his actions, which include murdering his own brother and fiancée to further his goals. His apparent caring actions, such as helping to rescue Kayo from her abusive home in the altered timeline, are purely pragmatic moves to maintain his cover and control over the situation. In the end, his development is one of unraveling composure as his carefully constructed facade is dismantled by Satoru. His notable abilities include genius-level intelligence, masterful manipulation, extreme patience, and a chilling capacity for violence, all of which he uses to evade capture for years and orchestrate complex cover-ups.