Jack 'Blackhawk' Ramsey is a freelance assassin.
Melissa 'Scarlet' Raines is an agent with the CIA.
The series is referred to as both 'Blackhawk', as shown by the use of that name in the title of each of the adventures, and 'The Scarlet Trilogy', because each of the three escapades deal with 'Hawk' as he is often called getting involved in matters that deal with her.
Blackhawk's history is unknown and that is just how this extremely dour, unsmiling, largely unfeeling man wants it. He has been in the killing-for-hire business for quite a few years and for all those years his reputation has remained exemplary. He prefers the long gun whenever possible and his signature method is a single bullet to the head. He will use a handgun if it is necessary and he is lethal with it as well but being a prudent and methodical man who wants to live past the latest contract, he prefers to plan his course, remove the target, and then be far away soon after.
What is not unknown is the fact that he routinely charges $1 million for a removal and we learn that, after his normal expenses and living costs, he has $89 million in the bank. Also known to a very few people is that while he is freelance and able to take assignments from wherever he wants, he has had as likely his best client the CIA. Of interest for more than one reason is the statement, "The only time the CIA ever reached out to Hawk for a job was when they didn't want to get their hands dirty, which was often."
Scarlet is a most unusual Agency operative. Besides being a regularly trained agent, she is also a highly skilled and proficient computer hacker. But what makes her unusual is that she works largely independent of the Agency, answering to no one per se but reporting at times to the Director of Operations. She is described to Blackhawk as "an information specialist we use. She has resources we don't have access to...She's one of ours. She's off the books, so to speak. Operating independently so as not to directly link her with the agency. Sometimes it comes in handy in cases like this."
The two people met more than three years before the events of the first recorded adventure and have remained in contact since then though usually at a distance. Now conditions have changed and they will need to rely on each other to survive.