Secondary characters can often be the most fun to write. There's a bit more freedom to make them fun and interesting, and less worry about all the backstory and flaws you need to give a main character. One of the things that's been most enjoyable with my own books is finding out which characters people like, and I'm happy that so far my secondary character Eskimo Sally (from The Day After Yesterday) is a favorite with some fans.
So without further ado, here are some of my favorite secondary characters:
Luna Lovegood in the Harry Potter series (books 5-7)
Dreamy, eccentric, and surprisingly wise, Luna is also just plain fun (highlights are her roaring lion hat, and her spaced-out commentary at a Quidditch game). Her whimsy is a good thing, considering how angsty the last three Potter books become. Her combination of goofiness and insight was a big inspiration to me when writing my aforementioned Eskimo Sally character.
Partridge in The Shipping News
A well-read, well-traveled, life-loving copy editor who befriends the friendless protagonist Quoyle and introduces him to the art of journalism, Partridge does what any friend should do - help a person out when help is needed. In addition to his friendship, Partridge helps Quoyle get a newspaper job in Newfoundland that helps Quoyle transform his life. The only downside is that Partridge isn't in the book enough (would have loved a chapter in which he visits Quoyle in Newfoundland).
Will Somers in The Autobiography of Henry VIII
Henry's fool Will is the character who "inherits" Henry's journal and comments on it throughout the book, providing his own perspective (particularly valuable when Henry disappears up his own ego) and filling in some blanks of historical perspective for the reader. It helps that Will is witty, sarcastic, and insightful, and his interludes in the journal are always welcome.
The Old Turk in Boys and Girls Together
William Goldman's underrated story of damaged souls is at its most heartbreaking with the story of Rudy, a kind child who's ultimately destroyed by his hideous parents. The one bright period in Rudy's life is when his family moves in with his maternal grandfather, a deli owner known as The Old Turk. He's a caring, sensitive, and sarcastically witty old man whose loving bond with his grandson is well realized and too, too brief.
Dr. Lilith Ritter in Nightmare Alley
All the secondary characters I've listed so far are nice people for the most part - ready for an evil one? Dr. Lilith Ritter is a psychiatrist who uses her brains and the information she's privy to from therapy sessions to help her erstwhile partner Stan Carlisle, a fake spiritualist minister. She's merciless and chilling, particularly when she robs Carlisle and uses what she's learned about him to drive the emotionally unstable man to a breakdown. And the best/worst part is that she gets away with it all.
Eustace Clarence Scrubb in The Chronicles of Narnia
I don't know if it's entirely fair to call Eustace a secondary character. He's secondary indeed in The Voyage of the Dawn Treader, but has a more substantial role in The Silver Chair, then is back to secondary in The Last Battle. But I'm including him here because he's arguably the only character in the series who has an actual arc. The Pevensie children start out nice and stay that way (well, except for that whole bit with Susan blowing off Narnia in favor of parties and lipstick, but that's a topic for another time). Eustace starts off as a smarmy little pill who's petty and nasty. He's turned into a dragon for a while, as a direct result of being a smarmy little pill, and after he's turned back into a human by Aslan, turns over a new leaf. The transformation isn't instantaneous and Eustace still has his douche-y moments, but overall it's quite a convincing and nice character arc.
Who are your favorite secondary characters?