The Strings of Purple.

When a villain is chosen by fate, but a hero is chosen by choice.

Being a villain isn't exatly the most entertaining, but Ariella Nebuelle doesn't get to make a choice. She merely does what she needs to do. She does what she can. Her best friend, Melanie Sauria, the hero, does everything in her power to stop Ariella's actions, but never Ariella herself. Alas, she continues to let Ariella do whatever she pleases, simply because she won't stop her. Their best friends for a reason, they don't even know why they are enemies.