Ran is a young woman in her teens. She has long black hair and brown eyes, like Tenma's. She was seen wearing modern clothes in a flashback to when she disappeared , but has since been wearing traditional clothes.
Technically not a blood member of the Oni (demon) Clan, Ran is actually Tenma's long lost sister from modern day Kyouto. She was summoned into Kyou by Akuram some time earlier, but was too frightened to be able to connect with Ryuujin and become Ryuujin no Miko. In her despair, she reached out instead to the black dragon Kokuryuu (黒龍?) and became engulfed by its power, erasing her memories of her past life and putting her under Akuram's control. Ran continues to battle with her alter-ego throughout both manga and anime, though perhaps more significantly in the manga. Probably on account of her association with the oni clan, Ran's name is also occasionally written in Katakana as 「ラン」 and features this way on the anime credits. However, in the manga it is given with its correct kanji, as shown above.
During the anime she is rescued from Akuram by Akane and the Guardians. She lost her memories of what had happened in the time that she had been missing, but regained the ones that she had lost when she was summoned by Akuram.
After her all her memories returned she left the protective barrier that was cast over the house in which she was staying, which meant that the demons could sense that she was there. Akuram later attacked her and Akane, stealing 3 of the talismans of the Gods as well as taking Ran with him.