The Code:05; she's hailed as one of the most powerful Code:Breakers. At first, Sakura thought she was a man, but she is actually a woman. Rui acts very much like a tomboy but is truthfully immensely shy. She is one of the few to know Shibuya's true identity and often uses this secret to blackmail him. Her addition to the Code of Hammurabi is "and the punishment of death for evil".

Rui's ability allows her to manipulate her shadow to defend or to attack by slicing others shadows (the damage done to the shadow becomes real). Her most powerful technique is "Empress Paradox", a shadow monster which envelops her body and can even eats up Rui herself. When Rui loses her power, she becomes a mermaid.