Mukuro is a master of the northeastern territory of Makai. She had hidden her identity of being a female for most of her life until the Ankoku Bujuutskai where she then revealed her gender.

However, she didn't always start from the position of a demon lord. When she was young, she was sold into slavery. Her master would constantly beat her, and rape her every year on her birthday. To escape his desires, she poured acid on her body, scarring half her form. This only angered her father he and had her imprisoned. Eventually, she killed her master and broke out, living off the streets beneath bandages and ofuda that covered her disfigured form.

In several years that followed a wake of killing and fear, she rose to her position and summoned Hiei to her side as her second-in-command and was given a position after he defeated her former aide, Shigure. The proposed tournament of the Ankoku Bujuustkai was to choose a new ruler for Makai. Mukuro didn't finish as first and was therefore relieved of her position. Her crew was then assigned to watch over Makai and guide back Ningens that wandered into it.

Mukuro confesses 'feelings' for Hiei after Hiei's battle with Shigure. Hiei's was unconscious and healing in a tank when Mukuro when Mukuro confesses her feelings.

INFORMATION FROM ABSOLUTEANIME.COM