If you're looking to get a unique listing of Main and Sub Characters, you can use the Distinct keyword:
SELECT DISTINCT MainCharacter FROM SomeTable
It would seem then that you would have some other control (a Combo or Listbox) which would list all Chapters in which the selected MainCharacter could be found. That would be done using a filtered RowSource in another combo or list:
SELECT * FROM SomeTable WHERE MainCharacter='" & Me.YourMainCharacterCombo & "'"
So you'd have cascading combos - the user would select a Main (or Sub) Character, and the next combo would be populated with data based on the previous selection.