Birthday: October 13

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 as a female. When Eriol transfers to Tomoeda Elementary, Ruby Moon created a false form named Nakuru Akizuki and enrolls in Seijou High School, entering the same class as Toya Kinomoto, Sakura's brother, and Yukito Tsukishiro, the false form of Yue who is unaware of his true nature. As Akizuki, she acts as a general annoyance, deliberately interfering every time Toya tries to talk to Yukito about the latter's dwindling energy. She claims she wants to consume Toya's energy herself, though it is never stated that she actually has a need for 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 is able to get past her due to his greater determination and desire to protect Sakura.

In the anime adaptation, she is voiced by Ryoka Yuzuki. In the English adaptation, Cardcaptors, both her true form and false form is called Ruby Moon and she is voiced by Willow Johnson.