Tatsuya is Kaoru's younger brother. He watches his sister play Super Mario Bros. 3, offering gameplay tips she finds irritating. Preparing a grocery list with their mother, he bounces with excitement when Kaoru discovers sparklers in a kitchen drawer. Later, in an auto junkyard, he voices concern about lighting the fireworks, though Kaoru reassures him. When a stray ember ignites nearby trash during their unsupervised activity, causing a fire, Tatsuya reacts with understandable fear. Mario and Luigi intervene to extinguish the flames, delivering a stern warning about fire hazards; during this, Tatsuya envisions a menacing skull-shaped flame, deepening his guilt and prompting his apology alongside Kaoru.

That evening, he resumes playing Super Mario Bros. 3 while a withdrawn Kaoru remains affected by the incident. Trying to comfort her, he downplays Mario and Luigi's warnings, inadvertently raising their mother's concern about their mental state. When their intoxicated father returns, Tatsuya nearly reveals the fireworks incident but stops after Kaoru's warning glance. During an earthquake, he joins the family panic but later spots his father's lit cigarette fallen onto the carpet. He alerts everyone, leading to the ember being stamped out, and explicitly reprimands his father for the carelessness.

After the earthquake, Tatsuya observes a fire erupting in a neighboring building. He stands with Kaoru and onlookers, witnessing firefighters Mario and Luigi rescue a mother and baby. Once the blaze is controlled, he approaches the brothers with Kaoru to affirm they remembered the fire safety lessons. Tatsuya has a gap in his teeth.