"...the only way to stop the calamity is: send the dead back to death."

- Katsumi Matsunaga

Katsumi Matsunaga is a Yomiyama North Middle School student from the 3-3 class of 1983, along with Reiko Mikami and Ryoko Akiyama. In 1983, he managed to successfully stop the calamity of Class 3-3; the only recorded case of the phenomenon stopping halfway through the year.

In 1983, Mr. Koga, the homeroom teacher of Class 3-3, organized a two-day and three-night trip to Yomiyama Shrine. The goal of the trip was to restore the abandoned shrine and pray to remove the curse. After praying, Mr. Koga declared everything would be okay and the class was free of the curse, much to the relief of his students. After leaving, the sky turned overcast, and it began to rain. Katsumi was walking behind Jun Hamaguchi and witnessed his death. After this, he got into a fight with a classmate in the forest. The argument intensified, and Katsumi punched his classmate; the classmate fell over and pierced his head on a tree branch, killing him. Katsumi, terrified, ran away and was questioned by the police who arrived at the scene. The next day, he returned to the scene, but the body had disappeared. When he asked his classmates about the classmate (his name can't be heard because the tape was altered by the calamity), no one remembers him, and Katsumi realized that he killed the Extra. Afraid he would forget his findings, he recorded his theories and confession on a tape for the following years' students to find and hid it in a classroom. But in time, even Katsumi forgot what happened on the class trip. After graduation, he moved to Tokyo to pursue his studies. He became a banker but quit after a few years and returned to Yomiyama to help with the family business. He actually works and lives in a hotel.

Katsumi has brown hair, brown eyes, and a negligent appearance. He is seen wearing a pale blue T-shirt, a dark blue shirt, a pair of pale pants, and dark blue shoes.

In the film, Katsumi appears briefly when Kōichi and Mei interview him about his year's Calamity. He wears the uniform of a hotel valet, and most notably, he's missing the fingers on one of his hands, a wound he self-inflicted for killing a classmate.

Katsumi seems to be an ironic and superficial person. When he comes out about the 9th Grade Class 3-3 curse, he becomes agitated and stressed.

Katsumi made his debut in "Sphere Joint" in Tomoka Inose's flashback.

In "Hair Stand", after he learns that Reiko Mikami was looking for him at the beach hotel, he goes out to look for them. On the shore, Katsumi had a chat with Reiko and the others; he and Reiko both commented on how long it's been since they've seen each other, and Katsumi told his old friend she hasn't changed much since Middle School. However, the fun at the beach is cut tragically short by Junta Nakao's death, despite the group's efforts to save him. Shocked out of his post-traumatic stress disorder, Katsumi remembered the tape's location.

At the beginning of "Glass Eye", Kōichi Sakakibara, Naoya Tashigawara, Yūya Mochizuki, and Mei Misaki listened to the repaired tape and learned how to end the Calamity: Send the Dead back to Death. Yūya later shared the tape with Izumi Akazawa and Takako Sugiura, which, unfortunately, caused more problems than it solved and led to most of 1998's curse-related deaths.