Riek Machar became the first vice president of South Sudan following its independence from Sudan in July 2011.
In July 2013, he was sacked by President Salva Kiir in a cabinet purge following a public struggle for power.
At the time of his sacking Machar said he would challenge Kiir for the leadership of the ruling party so that he could run for president in the 2015 election.
He was later accused by Kiir of masterminding a failed coup d'état on December 16, a claim that he denied.
"It was not a coup. Nobody wants that," he told Al Jazeera, claiming that he was "used as a scapegoat" by Kiir to purge the ruling SPLM party of rivals to avoid reforming it.
Machar has since retreated into the bush and acknowledged he is leading rebel fighters. South Sudan's government says he is directing rebels in Jonglei, Malakal and Unity states from his hiding position.
Machar hails from the Nuer tribe, which bitterly fought the country's majority Dinka tribe, which Kiir belongs to, in the early 1990s during Sudan's civil war.
Fighting for independence
From 1984, Machar was a rebel leader in the Sudan People's Liberation Army/Movement (SPLM/A), which spearheaded the war for South Sudan's independence from the north.
The movement was led by the late John Garang, who was respected by both northerners and southerners for his vision for a "new Sudan" - a Sudan that was united, secular and with equal rights for all.
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In contrast, Machar was always calling for self-determination for the south. After falling out with Garang in 1991, he split from the SPLM/A and formed his own splinter group, SPLA-Nasir.
A turbulent period followed, involving sometimes heavy fighting between Machar's forces and the SPLA. His groups received help from the Sudanese government and in 1997, he signed a deal with Khartoum and became an assistant to President Omar al-Bashir.
But a few years later, he returned to the rebels. He mended ties with Garang and rejoined the SPLA as a senior commander in 2002.
After the 2005 peace accord that ended the civil war and established southern autonomy, Machar became vice president of the South, maintaining this role after formal independence in 2011, until his sacking.
In his political career, he has made both Khartoum and the southern government suspicious that he is working for the other side.
As leader of the SPLA-Nasir, Machar was involved in the 1991 Bor massacre, where 2,000 people, mostly Dinka civilians, were killed. In 2012, he publicly apologised for his part in the bloodshed.
Machar was born in Leer, Unity State in 1953. He studied engineering at the University of Khartoum and was awarded a PhD in philosophy and Strategic Planning from the British University of Bradford in 1984.
In 1991 he married Emma McCune, a British aid worker during the civil war who was killed in a car accident while pregnant in Nairobi in 1993.
Their story became the subject of a book, Emma's War, and countless newspaper articles.
Machar is currently married to Angelina Teny, a prominent South Sudanese politician.