Alina Maschik is an agent with the CIA.
To be a tad less delicate and more to the point, she is an assassin with the Agency. In fact she may be in the top two they have if not numero uno. That distinction would have to be made by her and her friendly competition but since friendly is the key word here and the only real way to know who was better would not be so amicable, it remains a moot point.
Maschik is codenamed Viper and the epithet is extremely well suited. Like the poisonous snake, she can strike with frightening speed from out of nowhere and her "bite" is almost always fatal. If she wants a person dead, they become dead.
The expression "natural born killer" would not, however, apply to Maschik. Before her life with the CIA, a time ten years before the first recorded adventure, she was an average person, engaged to be married and ready to live a normal life. Then her fiancée decided to cheat on her and that ended that relationship. Then her older brother, a Marine serving in the Middle East, died in the line of duty. That changed Maschik and gave her a new purpose, namely to serve like her deceased sibling.
The military took her gladly and found in her extremely good aptitude in the types of things special operations required. They honed those abilities to a fine edge and made good use of her. That brought her to the attention of the Agency who knew a good (or "deadly") operative when they saw one. That is when Viper came into being and for the past few years, she has more than earned that name.
When we meet up with her, she is doing something she never expected or particularly wanted to do - she is returning home. Home to the jerk, now an FBI agency, who played around on her. Home to the two classmates who were like sisters back in the day, one now a bank executive and the other a Fibbie herself, partnered, believe it or not, with the jerk. Thomas Wolfe's line about not being able to "go home again" fits Maschik's situation for when going home after some time, the visitor finds home has changed, and so has the visitor. In the case of Viper, boy, has she changed.