Here are the characters who actually matter in the story (there are about a million characters, each with distinct personalities, but most of them are upper-class married rich ladies who only want to talk about who's getting married, and their old husbands who just want everyone to shut up so they can go shoot down some pigeons).
In case you couldn't tell by looking at him, Willoughby is a douche. He makes poor little fifteen-year-old hormone-bomb Marianne fall in love with him and then he abruptly runs away and marries someone else. Wow.
This makes Marianne sad. She becomes a depressed, crazy person. Then she and Elinor go to London. Yay London!
Elinor has to take care of her psychotic younger sister. Everyone feels bad for Marianne. Nobody cares about Elinor and her angst. Elinor likes Edward but she doesn't tell anybody.
Then Elinor makes "friends" with a vapid idiot named Lucy.
Poor Elinor. She's the only character I like. For some reason, though, Elinor continues to love Edward. While I seriously question his taste and his judgment.
Then everyone in the book mopes for a while. Marianne and Elinor leave London and try to go home, but Marianne gets so depressed she makes herself sick and has a giant fever and everyone thinks she's gonna die.
THEN. THEN, that jerk Willoughby shows up from nowhere drunk.
To which Elinor basically replies,
I almost forgot Colonel Brandon! This is where he becomes important.
Colonel Brandon marries Marianne. Then they all die because the planet Jupiter decides to leave its orbit and engulf the earth.
THE END. (I haven't actually finished the book yet.)