Separatist leader Alexander Zakharchenko will not be present at a summit on east Ukraine this week but his actions will help determine the success or failure of any peace deal.
While Russian President Vladimir Putin is seen in the West as holding the key to peace, Zakharchenko is regarded vital for ensuring Moscow's decisions are implemented.
The 38-year former mine electrician, who often carries a pistol and knife and usually wears combat fatigues, has been tightening his grip on power since being named prime minister of the self-proclaimed Donetsk People's Republic (DNR) last August.
A Donetsk native, he has succeeded where a succession of Russians parachuted in by Moscow failed — by bringing at least a semblance of unity and authority to the ragtag separatists.
"Russia dropped its maximum goal of conquering Ukraine and bringing (ex-President Victor) Yanukovych back to power," Yuriy Butusov, the Kiev-based editor of the Censor.net Internet portal, said of Zakharchenko's role.
"Russian operatives were no longer needed, so Moscow wanted a local man with enough influence and skill to unify and coordinate various battalions and their commanders," said Butusov, who has covered the region since before the conflict began and regularly travels there.
When he took over in the DNR seven months ago, the separatists looked on the verge of defeat by government forces.
Backed by what Kiev and the West said was an injection of Russian troops and weapons, the rebels turned the tide of the conflict and forced Kiev into talks and a cease-fire agreement last September even though the truce never fully took hold.
Moscow denies sending in arms and soldiers. But with Russia's backing Zakharchenko went on to win an election in November that was not recognized abroad and oversaw a rebel advance last month that pushed back Kiev's forces.
It was Zakharchenko who in effect declared the cease-fire dead, calling up new troops and saying the separatists would push government troops back to the edge of Donetsk region.
The new hostilities were key to France and Germany launching a peace initiative which resulted in Wednesday's summit being scheduled in Belarus with the Russian and Ukrainian leaders.
Kremlin Puppet or His Own Man?
Zakharchenko did not reply to interview requests. He has regularly praised Putin and congratulated him on his 62nd birthday, but has never said he takes orders from him.
Supporters see Zakharchenko as a tough leader they can respect.
Recent Russian television footage showed Zakharchenko continuing to talk to journalists after one of his bodyguards, standing just behind him, appeared to be shot by a sniper.
"We were dodging bullets but he didn't even flinch," said a rebel commander who gave his name only as The Ironside.
Zakharchenko rose to prominence when he and other men from Oplot, a street-fighting club, armed with old rifles and machineguns, stormed Donetsk city hall almost a year ago at the start of the rebellion against Kiev's rule.
He said they were restoring order, but Oplot took over local television transmitters and other property. By mid-2014 he was military commander of Donetsk, using Oplot's muscle to ensure control of the city.
He forged sometimes tense alliances with other rebel units and was wounded in the arm in battle, earning him a medal from ex-rebel leader Igor Strelkov, who commissioned him as a major. He also won three more medals, which he often wears in public.
As Russian-born leaders including his predecessor Alexander Borodai, Strelkov and deputy prime minister Vladimir Antufeev faded from the scene and left Ukraine, his rise continued.
Promising a better life while campaigning for the November election, in which he was the only major candidate, he said pensions should be "higher than in Poland."
"Pensioners should have enough to take a safari trip to Australia once a year to shoot kangaroos," he said.
He also told voters: "We are like the United Arab Emirates. Our region is very rich ... the only difference is that they don't have a war and we do."
For Kiev and the United States, he is little more than a Kremlin tool.
Samantha Power, the U.S. Permanent Representative to the United Nations, said Zakharchenko's statements — which he has sometimes reversed after speaking out of turn — failed to hide Moscow was pulling the shots in east Ukraine.
"Zakharchenko's statements are a problem for Russia because they are too straightforward," she said.
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