Haru Kasugai attends Eidai High School and belongs to their koto club. Standing at 191cm, he is the tallest character with a confirmed height in the series, sporting distinctive black hair that flares outward to the sides.

He prefers vegetables, tofu, and miso soup while avoiding meat and oily foods. During school breaks, he handles household chores like cooking, cleaning, and laundry, with a particular fondness for ironing. He also cares for his younger twin siblings.

In the koto club, Kasugai pairs with the notably shorter Sentaro Miya, forming a "Big Guy, Little Guy" duo. This dynamic evolves into a "tall-short-tall" configuration when Akari Kobayakawa joins them. Together, the trio competes in the national koto competition.