Reina Kisaragi is the younger brother of Shun Kisaragi, sharing a strong family resemblance that gives both siblings delicate, feminine features frequently mistaken for female. This similarity proves critical when Nagisa Tezuka's henchmen, pursuing a vendetta against Shinobu Tezuka, mistakenly kidnap Reina en route to visit Shun at Ryokuto Academy's Greenwood dormitory. They abduct him believing he is Shun, intending to use him as leverage against Shinobu. After the misunderstanding is resolved, Reina returns safely to his family. He maintains a composed demeanor throughout the entire ordeal.

Reina belongs to the Kisaragi household, which operates a chain of traditional Japanese inns. His immediate family includes his older brother Shun and an infant sister named Yui. His existence factors into Shun's backstory concerning inheritance dynamics within their family business, where leadership traditionally passes through female heirs.