Koumori, also known as Koumori-senpai, bears a name meaning "Bat." He carries a large black umbrella, frequently kept closed and wrapped around his body like folded bat wings. He wears a white button shirt, black trousers with brown tape wrapped around the ankles, and white sneakers. His hair radiates outward from his head.

A member of Beatnik, an elite organization from the "This Worlds" that investigates dimensional drifting—the sudden transfer of individuals into alternate realities—Koumori was tasked with judging Nagara, a student accused of causing the drifting phenomenon. After meeting him, Koumori declined to punish Nagara. He reported to the committee that Nagara was only one of many students sent to another world and defended him against unfair blame, promising no further unfounded tragedies.

On one occasion, Koumori traveled to Nagara’s island world, appearing as a closed black umbrella with legs, and lured him into a trap. This resulted in Nagara falling into an endless void and awakening in Babel, an inverted world that functioned as a massive pit. There, Koumori used his ability, "Upside-Down," to provide stability and hope to the students.

Through their interactions, it was revealed that Koumori had a past connection to Nagara: he was once a young boy who enjoyed spinning on monkey bars, an activity Nagara had dismissed. Despite this, Koumori held no resentment and was primarily following orders from Susugashira, ruler of Babel. After Nagara left Babel, Koumori continued to defend him publicly, refusing to let others blame Nagara for their drifting circumstances.