love interest vs sweetheart

love interest

noun
  • One who is of interest as a (potential) partner in love. 

  • A romantic relationship; a romance. 

  • A romantic plot or subplot in a film or book. 

sweetheart

noun
  • A person very much liked or loved by someone, especially when both partners are young. 

  • A person who is always very kind. 

  • A female member of a college or university fraternity. 

How often have the words love interest and sweetheart occurred in a corpus of books? (source: Google Ngram Viewer )