Her origin is even more mysterious than that of Nukata no Okimi; although a theory states that Kagami no Okimi was an older sister of Nukata no Okimi, there is no description in "Nihonshoki" to indicate that they were sisters, and thus no proof for the theory.
鏡王女の素性は額田王以上に謎に包まれており、額田王の姉という説があるが、『日本書紀』には二人が姉妹だという記述はなく、確証はない。