One who shares the work and the outcome equally. A partner is not a helper; the difference is who carries the cost.
Middle English partener, an alteration of parcener (a joint heir sharing an inheritance), from Latin partitio (a division), from pars (a part), via Old French parçonier; a partner was first one who shares a portion by right, and the -t- crept in by likeness to part.
One who shares both the work and its outcome, in full — the difference from a helper being exactly who carries the cost when it goes wrong.