Ruby Moon is one of two magical guardians created by Eriol Hiiragizawa. The counterpart to the original guardian Yue, Ruby Moon is genderless but prefers to act like a female. When Eriol transfers to Tomoeda Elementary, Ruby Moon creates a false form named Nakuru Akizuki and enrolls in Seijou High School, entering the same class as Toya Kinomoto.

Spoiler

She claims she wants to consume Toya's energy herself, though it is never stated that she actually requires it nor why she couldn't simply steal it. When Toya is finally able to speak with Yue and give him all of his magical energy to sustain Yue's life, Akizuki gives up her pursuit. In the final battle, Ruby Moon battles Yue and initially appears to be winning. However, Yue gets past her due to his greater determination and desire to protect Sakura.