Alex functions as a short form of Alexander or Alexandra, inheriting the Greek sense of "defender" while shedding some of the longer name's formality. Its brevity and unisex use have made it read as adaptable -- a name that moves easily between contexts rather than announcing a fixed identity.
Refuse the Moon in Cancer or Scorpio for marriages, unless it be in marrying of widows. - William Lilly (1602.-1681.)