The change, whether by design or due to the poor health of Anton Balasingham, puts SP Thamilselvan into the limelight as the rebels and government renew negotiations.
Sri Lanka has been wracked by civil war for 20 years in an ethnic conflict that has cost up to 60,000 lives.
"Balasingham was the moderate of the lot," said Lakshman Kadirgamar, a former foreign minister who led the peace process under the previous government.
"Those who have emerged now are the hardliners," he said of Thamilselvan, head of the political wing of the Liberation Tigers of Tamil Eelam (LTTE).
Thamilselvan earned his nickname because of the way he bobs his head and the way he smiles "just like a Cobra before it strikes", said one diplomat.
He returns to Sri Lanka on Thursday from Europe, where he studied a government proposal for a power-sharing body in the Tamil-dominated north and east.
The process did not involve Balasingham, an academic dubbed the "theoretician" of the party.
His life in London is thought to have made him more aware of international criticism of LTTE practices such as recruiting child soldiers.
Some see Balasingham's absence as proof of rumours he has been sidelined because of political differences with LTTE leader Velupillai Prabhakaran.
The Tamil Tigers want autonomy
in the island's north and east
Others said it was due to health problems connected to his diabetes and kidney transplant.
Thamilselvan's life has been inseparable from the LTTE and the reclusive Prabhakaran since, as a teenager, he went for military training in India.
He was a bodyguard for Prabhakaran and worked his way up the military ladder before switching to the political side.
Now 36, Thamilselvan needs a cane to walk since suffering a leg wound, fighting in the Jaffna peninsula where he was once the Tigers' commander.
In addition to his limited international experience, Thamilselvan's warrior life in the jungles of northern Sri Lanka, is thought to have kept him in tune with Prabhakaran.
"He does not have a mind of his own, he reflects Prabhakaran's thinking, he is Prabhakaran's man," said one
political observer in Colombo.
A secret government profile describes Thamilselvan as "a strong character and good at handling both military operations and political activity".
His hometown is Chavakachcheri in Jaffna, which was almost totally destroyed when government and LTTE troops fought over it in late 2000.
The two sides signed a ceasefire in February 2002 and started direct talks which the rebels suspended last April.
However, a Tiger response to the power-sharing plan is expected to lead to renewed talks in the next few months.