Jane’s Aunt Reed is sort of like a trial antagonist before the main action of the plot really gets going. She’s like the character version of tapping a microphone and saying "testing, testing, one, two, three." What’s important for Jane is to be able to recognize that Mrs. Reed is her antagonist, not just a powerful adult who’s right all the time. It’s no accident that the novel begins as soon as she figures that out: only when Jane realizes that Mrs. Reed is the bad guy (er, gal) can Jane herself become the heroine of her own story.
It’s interesting to think about Bertha as Jane’s antagonist – after all, Jane only sees Bertha twice, and neither time do they really talk to each other or anything (not that Bertha’s a big conversationalist anyway). But Bertha comes to represent everything that’s keeping Jane away from the man she loves, and Bertha’s weird nighttime activities are a passive-aggressive way of driving Jane away from Thornfield. Or maybe they’re an aggressive-aggressive way of doing it: tearing a wedding veil in half is pretty obvious.
Think of it this way: it’s the old versus the new. Nicole Kidman vs. Katie Holmes. Katie Couric vs. Kelly Ripa. It’s a catfight between the plain, dignified young Englishwoman and the crazed harlot from overseas! Ack! Yeah, we thought that would bother you. It bothers us, too. We think Bertha might be getting the short end of the stick on this deal, and there might be just a wee bit of prejudice against first wives, older women, and anyone who’s not English.
Wait a minute, you’re saying: Mr. Rochester is Jane’s love interest, not her enemy. But her enemy is exactly what he becomes when he tries to wheedle her into becoming his mistress. However he excuses or explains it, Jane knows that he’s trying to tempt her – and himself – into ignoring some basic moral principles, and she won’t let him get away with it. She’s made weaker in her struggle against him by her youth, inexperience, solitude, poverty, and financial dependence on him – but luckily she has more than enough sass to make up for all those deficiencies.
St. John is Jane’s final challenge. Finding out that Jane still has to fight St. John even after making the big decision to leave Mr. Rochester is sort of like finding out that Beowulf still has to face Grendel’s mother even after killing Grendel. (Don’t know what we’re talking about? Go and read Shmoop's coverage of Beowulf!) St. John isn’t just Jane’s antagonist because he wants her to marry him and she’s not interested; he’s also an extreme version of what Jane has been trying to become – a dispassionate, rational person guided only by calm consideration of morality and ethics. Jane has to reject him and embrace her own passionate nature before she can marry Rochester.