Who is Karla from Tinker, Tailor, Soldier, Spy and what is their importance?

Karla is the KGB agent, who recruited the mole named Gerald and functions as his control. According to Smiley, Karla began his career in 1936. During World War II, he regularly ran networks behind enemy lines. Karla may also have been in England for a period of six months, and he also spent some prison time in Siberia in 1948. He was arrested in India on immigration charges in 1955, and had just come from America. Karla is a Soviet master spy.