How can an idea be the antagonist? Think about it – what’s the main thing our protagonist Anne fights throughout the novel to get what she wants? While her family is certainly not on her side, even naïve, teenage Anne would have had the guts "to withstand her father's ill-will" (4.5). And while Lady Russell stood between Anne and Wentworth the first time around, she’s mostly out of the loop and therefore not interfering during round two. So the main thing that keeps our two lovebirds apart for eight years and 200 pages is that Wentworth is carrying around a mental image of Anne’s evil (well, spineless) twin and mistaking it for the real thing. Once Wentworth finally figures out that he’s been misjudging Anne all these years, the main barrier to their reunion crumbles.
Mr. Elliot is a slippery antagonist. He doesn’t seem all that bad when he shows up in Bath, and his main conflict with Anne is that he keeps turning up and giving the jealous Captain Wentworth the wrong impression of how much Anne likes him. When Mrs. Smith reveals his true character, however, he becomes an all-out villain, but more for his actions in the past than in the present.