Winky. Winky was so important to the plot of Goblet of Fire – her gorgeous (if misplaced) loyalty for her Master, Barty Crouch Snr, and his death eater son. Her adorable fear of heights. Her role in Hermione’s Society for the Protection of Elfish Welfare and her relationship with Dobby.
I adore Winky, and the other house elves, but I suspect it was down to budget that she couldn’t be used in the films.