The First was the leader of a group of Borg children who were stranded in space. Thier cube had become disabled, after the deaths of the adults on the cube. They were disconnected from the hive mind.
The First and his group had captured Kim, Chakotay and Neelix, and were holding them for exchange for Voyager's deflector. Seven tried to explain to the First that they were not going to be rescued by the Borg, since their distress signal was ignored by the Borg. The First became so enraged that he tried to assimilate Seven, but was stopped by the other children.
He refused to evacuate the ship when its warp core became unstable. He was shocked and died when he tried to fix the core. (VOY: Collective)