Billy was a lone Dark One child that Dias and the others spared long ago and she became attached to Marcus and Dias, following them around and watching them. Seemingly she changed her nature, though her nature remained chaotic evil and ever since then she has been up to all sorts of hijinks. Being given the name 'Billy' by Weapon II, Dias, Billy was taken in by Setsuna and Cassandra. However, despite being defeated and seemingly no longer an enemy, Billy has displayed amazing feats of powers, especially after the Dark Ones were completely defeated and she inherted their full power.

She had the might of the full force of the Dark Ones behind her and even inherited their power after their defeat, becoming one of the most powerful Dark Ones ever afterward. Billy's abilities seem almost limitless, limited only by her imagination and whatever tricks she deems. She can shape shift and change her age and appearance at whim, complete with a wide array of attacks and abilities. Though the shapeshifting power has been displayed by other Dark Ones later, Billy's talent has been unmatched due to her sheer strength.

The few times she has put forth evil agenda, one using Jennifer, the team has stopped her and appealed to her better nature - what little she has in her. She usually only listens to Freyja and Cassandra, looking up to them as mother figures.

Freyja created a tree for Billy to regain her powers, having rooted it to herself. Billy seemed to hold no grudge against her for failing to bring back Setsuna, and once the Erevis team began to search for the Progenitor ship, she joined on for whatever reason. Her goals cannot be guessed, other than perhaps she wants the technology for herself, having made deals with Loki to get the team closer to it, until finally she got aboard. As time went on, she began to display some of Freyja's light skills as a result of their connection, something that became more apparanty as the two became closer following Cassandra's apparant suicide.

However her dark nature began to become jealous of Freyja and Iceheart's closeness and was only saited when a future version of their daughter arrived - Elwyn. Elwyn and Billy became close when she learned Elwyn and a future version of Billy was close, having mentored Elwyn. Eventually she, and the future Billy, become a close trio and undercame many obstacles and Reikana and Jubei was born. And Cassandra and Setsuna returned to them as time went on, Cassandra's absense having been revealed.

Now though the Dark Ones are once again stirring.