The Chief of the Secret Intelligence Service serves as the head of the Secret Intelligence Service (SIS, also commonly known as MI6), which is part of the United Kingdom intelligence community. The Chief is appointed by the Foreign Secretary, to whom he directly reports. Annual reports are also made to the Prime Minister.

The chief of the Secret Intelligence Service typically signs letters with a "C" in green ink. This originates from the initial used by Captain Sir Mansfield Smith-Cumming, RN, when he signed a letter "C" in green ink. Since then the chief has been known as 'C'.

Sir Alex Younger has been the Chief of the Secret Intelligence Service since 2014.

More Info: en.wikipedia.org