Ideas

As far as Crystal's character goes, we had just discussed a clone from the CADMUS program, engineered for combat. She's got super strength and the ability to fly (like that of supergirl) as well as enhanced speed, agility and durability (spiderman, not flash) as well as some increased healing (similar to scarlet spider) she's in the dawn of some telepathic abilities...but that's a work in progress.
She's seen her fair share of combat and it's cost her a leg. Her missing limb is replaced with a state of the art prosthetic with a Judge Dredd style gun inside it (like Robo-Cop but with the availability to switch through several ammo types, including grenades) She also has goggles that have multiple vision sets (x-ray, thermal, night vision) and a full body suit with an active camouflage system that can change to any color through the use of nanites (allowing for a cloaking device)
The background that we were thinking was that she has seen a few years of combat for Cadmus, signing off on missions through different companies and subsets. When she started asking questions about what her missions purpose, and doing her own digging to find out more about Cadmus, she was black listed. A person that never technically existed, she remains an enemy only to those "in the know" and was later picked up by SHIELD. She's after finding a life of purpose when all she's known is combat and affliction...she's hoping that a new set of missions with a new lead will offer her something better...but it's hard not to be a skeptic.