Kei is the adopted daughter of Benkei Kuruma. She holds a secret about her origin due to a mental block that was present when Benkei adopted her.

It is later revealed that Kei's real name is Genki Saotome who was raised to be a boy but suffered from the loss of her family members and later blocked off all memories by the time she went into the underground shelter with Benkei giving her a new name. Kei's name and appearance comes from Kei Minamikaze from the Getter Robo Go manga.