Tetsuya is a brilliant but unfulfilled high school mathematics teacher who lives next door to Yasuko Hanaoka. Once a promising university student dubbed "Ishigami the Buddha" for his intellect, he now leads a lonely, highly regimented life. He harbors a deep, unrequited infatuation for his neighbor. When he hears a violent altercation between Yasuko and her abusive ex-husband, he steps in to cover up the resulting death, approaching the situation like a complex math equation to outsmart the police.

Yasuko is a divorced single mother who works at a local lunchbox shop, having left her previous job as a club hostess to escape her abusive second ex-husband. She lives a quiet life focused entirely on providing for her teenage daughter, Misato. When her ex-husband tracks her down and threatens her family, she acts impulsively to defend her child, thrusting her into a high-stakes deception under Ishigami's guidance.

Shunpei is a diligent and highly intuitive police detective with the Tokyo Police. Tasked with investigating a gruesome murder on the banks of the Old Edogawa River, he relies on his sharp instincts to pursue the truth. He frequently consults his brilliant friend, Manabu Yukawa, when cases stall. Despite Yasuko's seemingly ironclad alibi, his intuition keeps drawing him back to her as the primary suspect.

Manabu is a genius physics professor at Imperial University, affectionately nicknamed "Detective Galileo" by Kusanagi. Highly observant and analytical, he frequently consults on difficult police cases using scientific principles. His involvement in the current murder investigation becomes personal when he realizes the prime suspect's neighbor is his former classmate, Ishigami. He views the case as an intellectual challenge, testing his skills against the only person he considers his equal.

Misato is Yasuko's teenage daughter from her first marriage. She is deeply affected by the lingering threat of her former stepfather, Shinji Togashi. Her fierce loyalty to her mother precipitates the central crisis of the story, as she is the first to strike Togashi in defense of her home.

Shinji is Yasuko's abusive second ex-husband. Formerly a successful businessman, he lost his career due to embezzlement and subsequently developed an alcohol addiction. He tracks Yasuko down to extort money from her, initiating a dangerous conflict that sets the story's events in motion.

Kishitani is a junior police detective who partners with Kusanagi. He assists in gathering clues, checking alibis, and interviewing subjects. He occasionally questions Kusanagi's strong intuitions, preferring to rely on concrete evidence and straightforward deductions.

Kuniaki is a kind, recently widowed man who knew Yasuko from her time working at Club Marian. Upon hearing about her ex-husband's death, he reconnects with her to offer emotional support. His presence introduces a new, complicated dynamic into Yasuko's life during the investigation.

The Can Man is an older, unhoused individual who has lived in the tent city along the Sumida River longer than anyone else. He is a familiar fixture on Ishigami's daily walking route to school.

The Engineer is a neat, well-dressed unhoused man who recently arrived at the tent city. He keeps to himself, hoping his difficult economic situation is only temporary. Ishigami gives him this mental nickname based on his appearance and demeanor.