Fiona Fox was once captured and imprisoned by Dr. Eggman as a child, leaving her to fend for herself as a treasure thief.  She did abandon her old ways to join the Freedom Fighters and become Sonic's girlfriend.  However, it was revealed to be nothing but a facade as she was also dating Scourge, Sonic's evil counterpart.  As a result, she left the Freedom Fighters and teamed up with Scourge's Supression Squad and eventually the Destructix.

What I'm looking for in Fiona's voice is some kind of "bad girl" tone with a bit of sexiness to it.  While I prefer for her to sound more like a teenager, I honestly don't care too much about the pitch quality.  Just remember to make her sound evil, lol.