Inferior, underling, subordinate mean one, usually a person, who is lower than another.
Inferior, the most inclusive of these terms, may be applied to anyone that is lower in some significant matter (as rank, station, quality, or value) than another; the term suggests, explicitly or by implication, a comparison, sometimes with those obviously and individually higher or superior but sometimes with those merely belonging to a level felt as higher.
Underling routinely implies subjection to the will or wishes of another; it may apply to a condition from that of a slave or servant to that of one who is just below the master but ordinarily it implies some degree of contempt.
Subordinate likewise suggests subjection but has an entirely different flavor from underling. The term implies a being in subjection to the will or wishes of another but seldom carries a suggestion of disdain and usually expresses no more than relative position in a hierarchy.