Citing the Universal Declaration of Human Rights, Ban has been a champion of equality, regardless of gender and lesbian, gay, bisexual, transgender, questioning or queer and intersexual orientation.

He oversaw the establishment of UN Women, putting UN agencies affecting women under one umbrella for women's empowerment, appointed the first envoy for youth, focused more on prevention of natural and man-made disasters, advocated stopping the flow of illegal arms as the world faced mass migration.

"Ban has at least been willing to criticize his own performance, and he has improved as secretary-general over time," Gowan told Xinhua.

Ban, a South Korean national who loves Chinese classic philosophy and calligraphy, visited China for 11 times as the UN chief, and met on various occasions with Chinese leaders, who lauded his active role in enhancing the cooperation between the United Nations and China over the past decade.

Ban is leaving, not unusually, a host of unsolved issues for his successor. In addition to the aforementioned, they range through conflicts, mostly in Africa, and most recently the bloodshed in South Sudan, to terrorism, xenophobia, the seemingly endless quest for UN reform, demanded by the membership, which sometimes thwarts its progress because of special interests.

Asked at the Council on Foreign Relations in New York earlier this month what his biggest problem was, Ban replied, "The lack of empathy and lack of compassion of the world leaders," blaming them for "focusing on their very narrow personal or regional interests."

People express their grievances and leaders react by using "military and police forces," he said.

For Guterres, Ban has published a briefing book, for his new role. It's 345-pages long.