Rui (塁) 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.
"The 'Sixth Flame Dragon' with wild hair. Her 'illusory flame' creates illusions from her wielder's thoughts. Her human form is a voluptuous woman with long black hair."
As a human, Rui is a beautiful kunoichi who is not afraid to show her body to seduce her enemies. She has gray eyes and smooth dark purple hair. She wears a loose light pink robe that revealed a good amount a cleavage and has a purple obi tied on her robe. She also has three gold bracelets on her right wrist. She has a rose tattoo above her left breast in both the manga and anime.
In the anime, Rui has black hair and olive green eyes. Her main outfit has a minor color change; her loose robe is lavender and her obi is dark brown. The rose tattoo appears to be black and her bracelets are now two instead of three as the color is white silver.
In her dragon form, Rui has blue eyes and has rather wild 'hair' and the color of her form is orange. Her voice became distorted and appears somewhat masculine which both Recca and Kashamaru mistakes her as a man during their battle.
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.
- In the English Dub, Rui is mistaken as a man while she reveals herself on her dragon form during the battle between Recca and Kashamaru.
- Like Raiha and Reiran Katashiro, Rui's name is changed from R to L words during the English Dub as her name becomes Lui, Laiha and Leilan respectively.
- Kashamaru also calls her as Louie due to her voice became masculine while in her dragon form.