Rui is the sixth Flame Dragon that is able to create illusions right when Recca thinks of it instantly. Her hobby is creating riddles. She demonstrated the ability to create a flame pillar during Recca's fight with Kashamaru.
As a human, Rui is a beautiful kunoichi who is not afraid to show her body to seduce her enemies. She has grey eyes an smooth black hair. She wears a loose robe that revealed a good amount a cleavage. She has a rose tattoo above her left breast in the manga.
As a dragon, Rui has blue eyes and has rather wild 'hair'.
Rui enjoys teasing people usually by either showing a good amount of skin and cleavage or by confusing them with riddles or changing her appearance. Especially Recca when she tries to tease him to seduce her as the latter bleeds his nose continuously when he looks at her appearance.
Rui is able to create illusions the second Recca thinks of it. She can make his opponent relive his or her nightmares or make him live his worse fears, effectively making her a mistress of illusions and hallucinations.
As a human she had the nickname 'Katanashi no Rui' (Rui the Formless), due to constantly changing her appearance and mannerisms so often that her true appearance was forgotten by everyone.