Trading mercy.

"Mercy!!" Alix exclaimed. Her eyes were wide, mostly in disbelief. She opened her mouth and laughed loudly. It was unbelievably surprising the things Billi said and did. In what way had she shown her any mercy? Billi had stabbed invisible daggers into every part of Alix's body all through the years. 

She even as far as to poach any talented students of Songbird music academy when Alix had just started out, recommending them to other schools. 

She framed Alix for a lot more things over the years with the goal of ruining her reputation. Whenever Alix was down in the mud, she would show up with a smile and offer some little money for one of Alix's compositions, which Alix always rejected. Billi's goal had always been and would always be to watch Alix squirm like a worm beneath her feet.