Sara X is an assassin.
That is the tradename she uses in her line of work which is killing people for money. The kinds of people who hire her are invariably well-off enough to handle the usual fee of $100k per victim as well as being desperate enough to get rid of someone they will risk it. When we first meet her it is said that sports figures were frequent customers of hers as newly rich athletes either have existing spouses who will not let go of a now-wealthy 'loved one' or they have girlfriends who have become bothersome. $100k to Sara X and the problem go away and almost always in a way that escapes the interest of authorities.
Normally assassins who work the private sector like Sara X does would not be of interest in this collection but one of her previous customers has become the leading candidate for President and he and his powerful father-in-law, head of a major security firm, are worried about the past returning so they use all the impressive resources available to track her down and even though she has retired by this time and started a family of her own, they strike, her husband and young son are murdered, and she returns to her previous line of work with a vengeance. Except it is no longer sports figures who incur her wrath but political ones.
We learn that she was born Janet Mills. No living relatives after her parents died in a car accident while she was in college. According to the info the security firm learned after a lot of investigation, "She worked for a while as a phlebotomist, then on the IV team for a major teaching hospital where she apparently got her training. She became extraordinarily proficient in hitting veins in even the smallest neonatal cases and was highly thought of by her co-workers. Apparently she'd gotten involved with a married anesthesiologist while working at the facility. We believe it was around then that she committed her first killing. The wife of the anesthesiologist died of an apparent heart attack while he was out of town at a convention. Janet then apparently broke off the relationship with the anesthesiologist. She'd come into a substantial amount of money in some manner as she took a very expensive vacation. When the vacation ended Janet Mills vanished. She's never used that name again. We then had a series of untimely deaths of spouses of people associated with the anesthesiologist."
After that her new career really took off resulting in, again in the opinion of the security firm, a definite "39 deaths and strongly suspected in over 100 more". It was suspected she had stashed upwards of many millions of dollars away. And then she stopped.
And that is when Shannon Brewer came into existence, happily married to Jack and the mother of toddler Tyler. A new life. Until the security firm finds her and the candidate and his father-in-law order her death and she survives but Jack and Tyler do not.
And Sara X is back for revenge.