SLOVYANSK, Ukraine — The rebel leader spread a topographic map in front of a closed grocery store here as a Ukrainian military helicopter flew past a nearby hill. Ukrainian troops had just seized positions along a river, about a mile and a half away. The commander thought they might advance.

He issued orders with the authority of a man who had seen many battles. “Go down to the bridge and set up the snipers,” the leader, who gave only a first name, Yuri, said to a former Ukrainian paratrooper, who jogged away.

Yuri commands the 12th Company, part of the self-proclaimed People’s Militia of the Donetsk People’s Republic, a previously unknown and often masked rebel force that since early April has seized government buildings in eastern Ukraine and, until Saturday, held prisoner a team of European military observers it accused of being NATO spies.

His is one of the faces behind the shadowy paramilitary takeover. But even with his mask off, much about his aims, motivations and connections remains murky, illustrating why this expanding conflict is still so complex.