The son of refugees who fled North Korea during the 1950 -53 Korean War, Moon, 64, grew up in poverty and spent time in jail for protesting military-backed dictators. Moon later became a human rights lawyer and worked for late liberal President Roh Moo-hyun.
Moon's North Korean parents resettled in South Korea's southeast before he was born in January 1953. They initially lived in a POW camp.
As a boy, he often went to a Catholic church with a bucket to get free US corn flour and milk powder.
In 1975, Moon was jailed for months for staging anti- government protests before being conscripted into the military's special forces.
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Moon became a lawyer and joined Roh's law office in the early 1980s. They defended the rights of poor laborers and student activists until Roh entered politics in 1988.
Moon says their friendship changed his life.
After Roh became president in 2003, Moon became what local media called Roh's "King secretary" or "Roh Moo-hyun's shadow."
Moon says Roh's death led him to politics. Moon lost the 2012 election to Park Geun-hye by a million votes.
When a corruption scandal involving Park flared last fall, Moon saw his popularity rise amid massive public outrage toward her conservative government.