Ross and Lyle are agents with the DIA.
A Spoiler warning! In order to explain an important fact on the identity of one of the characters, it is necessary to reveal something that happens halfway through one of the books for without that explanation, it is impossible to describe the character.
Martin Ross is an aging professor of computer science at a university, 58 years old in his first appearance in Fallback. He has been married to Elaine, a fellow computer whiz and teacher, for 35 of those years. He appears in moderate physical condition though the paunch over his belt shows the last few years have not seen much exercise beyond walking. Ross is extremely intelligent and well-read and he speaks several languages including knowing Russian like a native. When he is not teaching, he is a member of the Institute for Defense Analyses specializing in computers and his advice has been sought on numerous occasions by the NSA.
David Cane is an agent with the DIA. He is fairly newly married with a baby son very much keeping him up at night. His work out in the field is considerably more dangerous but at least he can usually sleep which is more than he can with the newborne. Cane is at most in his early 30s and has been an agent for several years. He loves his wife and child deeply but loves his country even more which is why he still volunteers for dangerous assignments.
Cane and Ross are chosen, in that book, to go behind the Iron Curtain to find and destroy a new computer system which has the ability to launch a nuclear first strike against the West without the mutually assured destruction that has kept both sides from starting a war. This dangerous and likely very physical mission requires a man who is not only fluent in Russian and in computerese, but also extremely fit and able to take care of himself. Cane agrees to train Ross and get him into shape and in the few weeks that it takes to do this, the two become close friends. Both are devastated when it is learned, just before the mission, that Ross has terminal stomach cancer and can die at any time.
The Director of the DIA knows that without Ross the mission is a failure and without the mission the chances for the U.S. to survive is virtually none. He knows that Cane has the physical ability to pull it off but not the necessary computer knowledge like Ross has. He also knows a doctor who has been experimenting with brain transplants....
Clive Lyle is an agent who is incredibly handsome and devastatingly charming. He definitely enjoys the attention of the ladies and thinks nothing of availing himself of their charms and then moving on. If the mission calls for it, he also is apparently willing to apply his suave manners to his own sex. Lyle is even more competent at killing than he is at love making and he is just as ruthless. Getting the job done is something Lyle is very good at because he lets nothing get in his way.
David Ross is an agent who has the body of David Cane and the mind of Martin Ross. This "individual" become partners with Clive Lyle in the second half of the second book and remains that way in the third.