Mamiya Takuji

Yuki's classmate, Takuji, is the protagonist of "It's My Own Invention" chapter. He is a reclusive boy from an otaku subculture who's always stammering while speaking with others. Takuji rarely attends class as he spends most of his time secluded in his secret hideout that he had constructed under the school’s old drained swimming pool.

Spoiler

His personality will go through a profound transformation after the death of Takashima Zakuro.