East German, son of a doctor who had tended Willi Brandt's wounds and sheltered him. SSD (East German Intelligence) recruited him and had father write letter to Brandt asking him to help son. He became a special assistant to Brandt, worked long hours and made himself indespensible. His access to sensitive information grew as Brandt became more important. Not exposed until 1974 by which time he'd done irreparable damage, especially to Brandt's reputation.
Guillaume and wife were arrested, tried, and conviced. Later traded for West Germans and now lectures in KGB schools.