Kasuga was born in Kasukabe, Saitama. He has a younger sister that, according to him, resembles Ayumi Tanimoto, a judo athlete. His parents met at their work place. He was liberally raised, called "the cute one" in a privileged family.
With excellent reflexes, he learned swimming from his early childhood through his junior high school years. He attended a private junior high school in Tokyo, Nihon University Dai-ni Junior High, and met Wakabayashi there when they were 8th graders. Wakabayashi, a trouble-making kid sitting just behind Kasuga, played with him, sneakily cutting his hair from behind without telling him. In spite of Wakabayashi's misbehavior, Kasusga's first impression on Wakabayashi was that he was a "cute boy," and they got along.