Lars Christopherson, aka Lars the Hitman, is a freelance agent.
Fredric Ulrich is an agent with the German BND.
Christopherson gets series title credit here because he is given that by the author and/or publisher and who am I to argue. I would point out, though, that in the first recorded adventure, Lars does not make an appearance until a quarter of the way through. Fredric comes into the action far sooner. Both, though, are vital to the action and while they are not partners in any way, they will make a pretty good team when life and situations force them to cooperate.
Ulrich is a member of the impressive Bundesnachrichtendienst, or BND or Federal Intelligence Service of the German government. The description given of him has him being 5'9 "with an athletic build, 175 pounds of body weight, and deadly fighting skills. He didn't have a great deal of visible muscle mass. But long hours on his white, carbon-framed racing bike made his legs strong. Any of them closely observing him would notice it in his strides or spy the outline as he walked away." He is definitely one of the best field operatives that the BND has but his relationship with his immediate superiors is not one to admired. Still, he is extremely patriotic and devoted to his work.
Christopherson is, as his epithet indicates, is a hitman who makes a killing - em - killing. The second adventure starts out with him negotiating his next assignment being offered $250k for a hit, demanding $500k for it, and settling almost immediately at $350k because the contract looks like a very simple, easily done gig. He is described as having a runner's physique (sensible since we meet him as he concludes a 7-mile run for exercise) and a "lean face, framed by wavy blonde hair". But he is more than just a hired gun as he is also a skilled investigator who was trained well when he joined the Department of Homeland Security and had a pretty good success record in the few years he worked there. His transition from a federal cop to an accomplished assassin is an interesting one.
Ulrich and Christopherson do not know each other when the series begins but they will over a bit of time come into contact and will after a bit more time elapses learn that survival will mean trusting the other literally with him life.