Almost nothing is known about the true identity of this woman, capable of transforming into living smoke to evade capture or penetrate any but the most secure buildings or enclosures. She was, at one time, an assassin, with a reputation for reaching targets very few others could touch. Then--for reasons as mysterious as her origin--she changed her modus operandi completely, going from killer-for-hire to just plain killer.
No one knows how she chooses her targets, except they appear to be predominantly in law enforcement--local and state police, federal agents, PRIMUS and UNTIL agents--and superheroes. A favorite tactic of hers is to stalk her target until they come in conflict with a criminal or supervillain, then kill the target's opponent as the battle ensues, thereby "saving" the target. She will then confront the target days or even weeks later and say this--
"You owe me your life. I'm here to collect the debt."
No one knows when she will stop--or even if she can be stopped. All law enforcement can do is wait for her next strike.