Considering season 8 retconned the Trix to only be sisters in friendship and not biological triplets, this leaves room to explore both Darcy and Stormy and their motives and backstories.
Stormy is said to be the most powerful of the Trix however...she never really feels like that. So here's an idea for Stormy's backstory.
Stormy was born a princess to a tiny peaceful kingdom Typhun her home planet was beautiful and covered in flower filled farms and plains. When Stormy was born, a powerful dark surge had flashed across the land. This caused the King and Queen and their royal advisors that Stormy had been cursed. Stormy was raised in strict and hard supervision, when she was young she would constantly be scolded and punished for simple things or tiny mistakes. The Royal Family thought raising her this way would force out any darkness in her heart. However it had the opposite effect, she grew up to be rebllious, sassy, yet free spirited. Upon her 13th birthday Stormy's parents had surprised her by stating that because she was becoming too "out of control" that she would be removed from the palace and placed in an isolated tower, where only her handmaiden and private tutor would visit her. Upon hearing this news it angered Stormy, triggering her latent Witch abilities. This event caused Stormy's power to burst around her, it was uncontrollable, it was a force of nature. The lush, peaceful, and gentle lands on Typhun were covered in dark clouds. Tornados began to travel around the countryside, tearing apart everything in their path. The world was covered in a never ending hurricane. Stormy had unknowingly Cursed her own home planet. This curse lended itself Half of Stormy's full power. Because of this curse, Stormy had always been fighting at half of her full potential. The storms were so bad that the peaceful and farmland kingdom of Typhun was destroyed, the entire population had to retreat underground and restart their lives in caves. After this event, Stormy had been contacted by Headmistress Griffon, as she had nowhere else to go Stormy moved to Cloud Tower. Where she met her two closest friends.