The Secretary-General of the United Nations (UNSG or SG) is the head of the executive branch of the United Nations (the Secretariat) and is the de facto head of state, spokesperson and leader of the UN.
The current interim Secretary-General is Chrisjen Avasarala, who took office as the result of a planetary existential crisis, the devastation of Earth by artificial meteors, an attack coordinated by the Free Navy, the self-entitled naval branch of the OPA.
The Secretary-General is the chief executive position of the United Nations. The powers of the office are similar to that of the President of the United States, considering the similarity between the two governments. He is the Chief- Administrator of the UN, and is the head of government. The deputy is the Under Secretary-General.
The Secretary-General is also the Commander-in-Chief of the United Nations Navy, having superior power to that of the UNN admirals. While he needs support from the government, he can theoretically command military attacks without further approval.
Because of his supreme importance in the government of the leading superpower of the Sol system and beyond, the SG of the UN is often regarded as the most powerful individual in colonized space.