alter ego vs counterpart

alter ego

noun
  • Somebody's alternate personality or persona; another self. 

  • A very close and intimate friend. 

  • A corporation used by a person to conduct personal business in an attempt to shield himself or herself from personal liability, and which a court may penetrate by "piercing the corporate veil" to impose liability on the person when they commit fraud or injustice. 

counterpart

noun
  • One who or that which resembles another. 

  • Either of two parts that fit together, or complement one another. 

  • A duplicate of a legal document. 

  • One who or that which has corresponding functions or characteristics. 

  • Either half of a flattened fossil when the rock has split along the plane of the fossil. 

verb
  • To counterbalance. 

How often have the words alter ego and counterpart occurred in a corpus of books? (source: Google Ngram Viewer )