A snake demon used to seal Pai's consciousness and possess her body in 3x3 Eyes. Despite this, she is not actually evil.
When Benares sought to put the Sanjiyan Parvati under his control he chose his servant, a snake demon named Houasyou, as his implement. While he was successful in fusing Houasyou onto Parvati's body, suppressing her consciousness and giving her control of the sanjiyan's body, things didn't go completely according to plan as Houasyou lost her memory and Benares was temporarily destroyed by Parvati's counter-attack, preventing him from rectifying the situation.
The Houasyou-possessed Pai was taken in by an elderly Japanese couple and raised as a normal girl, totally oblivious to her past until Yakumo Fujii found her four years later. Unaware of Houasyou's existence, Yakumo merely thought Pai was amnesiac, and brought her to Tibet in the hope of restoring her memory. From Tibet, they went to the Sanjiyan Holy Land, where Benares revealed the truth of Houasyou's existence to her. Fearing that giving up Pai's body would mean losing the normal life she'd enjoyed as well as losing Yakumo, Houasyou was reluctant to release her. However, watching her friends sacrifice themselves to protect her, she realized the value of selfessnes and abandoned the Sanjiyan's body, allowing her to defeat Benares.
Presumably through the Sanjiyan's help, Houasyou was able to take human form and resume the life she'd lead while in control of Pai's body, although she was now known as "Yoko" and oblivious to her past once more.
While Sanjiyan and Yakumo were happy to leave her in peace for the most part, they were forced to disrupt Houasyou's life and seek her help later in the series, and Houasyou became one of their most constant allies.
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