Mr. Koga (古賀, Koga) was the homeroom teacher of 9th Grade Class 3-3 in 1983.

Mr. Koga is an adult with short black hair and black eyes. He is shown wearing a white shirt with a black tie and grey pants.

In 1983, Mr. Koga was the homeroom teacher of Class 3-3. In a last, desperate attempt to break the class, he organized a trip for two days and three nights at the inn.

After praying at the shrine, Mr. Koga declared everything would be okay and the curse had been lifted, much to the relief of his students. This was not the case, however, as Jun Hamaguchi and Yuki Hoshikawa died on the way down the mountain before Katsumi Matsunaga stopped the calamity by accidentally killing the extra student. However, as the dead student was immediately forgotten by nearly everyone, it was assumed by Mr. Koga and the rest of the faculty that his plan had worked.

Because of this, in 1998 Ms. Mikami organized the same trip, hoping visiting the shrine would fix the curse. Instead, it had much the opposite effect, as two students took matters into their own hands and the innkeeper burned the inn to the ground, killing several students.