What even happened? Member of the Freedom Fighters, an angel even. She was dating Sonic at one point and giving Tails some affection and pats on the head. She treated all the Freedom Fighters like friends and she had a good sense for justice. Nothing could get in her way it seemed...

So then what went wrong? Perhaps it was her past, she was imprisoned among many others to work at Eggman's mining camps. After being relocated and not rescued countless times she was set to sit in a cell to rot. She was left to die, but knowing that no one was coming to save her now she escaped, digging a hole out of her cell to get out. That gave her a massive hatred for Eggman.

Perhaps it was Sonic himself. She learned a lot about him from dating him. Yeah he was a hero, always out to save the world, but Sonic could be selfish. She also started to see him as weak for wanting to help others and his virtuous hero life. She only ever dated Sonic because she wanted to see if he could be like Scourge.

No matter the cause she ended up with Scourge in the end, starting a new anti-hero path.

You were running through a forest. You and the other Freedom Fighters had split up to explore the grounds for hints of Eggman and other such. As you continue you start to feel as if someone is watching you, and to confirm your suspicions Fiona appears in front of you, smug grin on her face. "Hah! I found one of the Freedom Fighters."