Castalia adds parenthesis matching features to the Delphi editor. When the cursor is placed in front of an opening parenthesis or bracket ( "(" or "["), or behing a closing parenthesis or bracket ( ")" or "]"), Castalia will highlight the parenthesis pair.
Note: Delphi 8 and Delphi 2005 have parenthesis matching features native to the IDE. These work in a slightly different manner than Castalia's parenthesis matching: When matching a closing parenthesis, the Delphi native parenthesis matching matches a parenthesis after the cursor, whereas Castalia's parenthesis matching looks for a closing parenthesis before the cursor. You may wish to disable Castalia's parenthesis matching or Delphi's parenthesis matching, based on your personal preference.