Who is Georgi Kramarenko from The Gulag Archipelago, 1918-1956 and what is their importance?

Solzhenitsyn's cellmate in Lubyanka Prison Cell 67 and later 53, Kramarenko is constantly trying to obtain tobacco. Solzhenitsyn finds something "alien" about him and clams up. Later, he learns that every cell must have a "stool pigeon," who trips an innate alarm. This helps Solzhenitsyn survive 17 years of sharing dangerous thoughts with people without a misstep.