Who is Thomas Chelgrin/Ilya Lyshenko from The Key to Midnight and what is their importance?

Ilya Lyshenko is a Russian who has joined a project code-named Mirror. As part of this project, Lyshenko will take over the life of an American, become rich, and get involved in politics in order to leak secrets to his homeland. When Thomas Chelgrin becomes a prisoner of war in Vietnam, his uncanny likeness to Lyshenko proves to be deadly. Lyshenko is given extensive plastic surgery to make him look even more like Chelgrin and then taught everything there is to know about Chelgrin. Finally Chelgrin is killed and Lyshenko assumes his identity.