All are battling evil. I think the characters who confront it most directly are Frodo and Sam, who endure Sauron's tempting influence constantly in the form of the Ring and its effect on those around them.
Though not as clear in the movies, Aragorn loves Arwen and Elrond decreed that no man less than the King of Gondor (AND Arnor, in the books) will wed her. Aragorn sees his destiny before him and is rushing to it. If he succeeds, all else pales before Arwen, who must accept a mortal doom. If he fails, nothing else matters after the loss of Arwen. He is battling Evil, certainly, but Arwen is his over-riding desire. Like I said, it's sometimes hard to tell that from the films.
In the books, Aragorn recognizes the events that unfold around him as signs of a destiny being fulfilled and gains in confidence. In the movies, he's still a 20th century man plagued by doubt up to the eve of the battle of Helm's Deep. Will he still be that sort of man when ROTK opens?
That is the part most irksome to me, but I can live with it.