Mike Walton is an agent with the IMSI.
He isn't either when we first meet him. At the beginning of the first recorded adventure, he is Mike Powell, a sergeant with the Royal Canadian Mounted Police working as a member of the federal air marshal program. He is happily married to Dr. Lisa Powell who is very pregnant with their second child. Powell is a dedicated man both to work and family. Then a horrific terrorist attack sponsored by the name known only as the Sheik took most of his life away. While Powell and team were trying to stop one attack at a airport, another at a train station took many of his loved ones: his in-laws, his mother, his daughter, and his unborn child. His wife survived to deal with the anguish. Powell himself was severely injured, much of his face destroyed, and his partner was killed.
Several weeks later, coming out of the induced coma he had been in, Powell learned of his loss. He also discovered that to the rest of the world, he and Lisa were also among the lists of the dead.
Lisa had, of course, not been in that coma and during the time her husband was unconscious, she had agreed to start a new life as Lisa Walton, married to Mike Walton. She also agreed to become a member of a private group doing its best to stop such attacks and to surgically excise terrorist cells without worry about political ramifications or international boarders.
That group was IMSI, which stands for the International Market Stabilization Institute. With a name like that, it was hardly the sort of organization one would picture fighting terrorists and in actuality, much of the group did exacly what its name implied. According to the founder as he explains things to Walton upon his awakening, "IMSI is a real eighteen-month-old company doing real foreign-market analysis for nine of the biggest corporations in the United States. You can find the company and its New York headquarters telephone number in the Yellow Pages although we're not able to take on any new clients at the moment." Naturally, all that is but a front for the real purpose.
At the time they start work for IMSI, the Waltons are in their late 30's and have enough anger and hatred and determination to do the sort of things that IMSI wants to accomplish.
And while Mike Walton is the man that we will follow most out in the field taking the fight to the enemy, Lisa's keen intelligence and her unwavering determination make up for a lack of actual experience as she remains a vital aspect of the partnership.
And then there is Zima. You need to read the books to learn about Zima but she is worth it just by herself.