The Soviet double agent who provided intelligence to the United States during the Cuban Missile Crisis was Oleg Penkovsky.
Penkovsky was a senior Soviet military intelligence officer in the GRU who secretly passed information to Western intelligence, working through both American and British contacts. His documents described Soviet missile capabilities, military procedures, and the condition of Soviet forces involved in the Cuba deployment.
That intelligence helped U.S. analysts interpret aerial photographs of missile sites and assess whether the Soviet Union was prepared for a prolonged confrontation. Penkovsky’s information was important, but it was one part of a larger intelligence effort that also included U-2 reconnaissance flights and other sources.
The KGB arrested Penkovsky in October 1962, during the crisis itself. He was tried for treason and executed in 1963. Julius Rosenberg, Aldrich Ames, and Kim Philby were also associated with espionage, but none is the canonical answer to this question.