Gérard de Villefort in The Count of Monte Cristo. He's basically the alt protagonist of the story, like in the Korean movie based on it, Oldboy where the roles are reversed. Although his son Andrea is a pretty funny imp, too.
Wilkins Micawber in David Copperfield. Pretty much a trope (still moreso than the "I'll eat my head" guy in Oliver Twist) but it never gets tiresome.
Barnabas in The Castle. Kafka didn't give much weight to the physiognomy of his secondary characters before this novel, and surprisingly here is a very bizarre 'creature' who is even more naive than his main characters.
Madame Merle in The Portrait of a Lady. Every smirk, sneer and tick of hers is recorded so convincingly by Henry James that the reader can basically tell what she looks like.
Natasha Rostova in War and Peace. Although she doesn't factor into any of the serpentiform war tableaux, she's by far the most fleshed out of all of Tolstoy's characters.
The duke and the dauphin in Huckleberry Finn. Can't remember which one is more ridiculous, and they even would have gotten away with the fraud, were it not for Huck being more rascalous than them.
Phaedra, Racine's title character. Though it's a play, not a book. How can someone be this ruthless, desperate and at the same time tender?