Computed field that displays the full username of the author
Not 100% clear what your use case is, but it's possible that the realname module will do exactly what you're after https://www.drupal.org/project/realname 

under /admin/config/people/realname enter tokens for the fields you want eg [user:field_profile_first_name] [user:field_profile_last_name] then whenever the username gets printed it will display these fields instead