Favorite secondary characters? (Note the plural)
The Dowager Duchess of Denver in various Lord Peter Wimsey stories by Sayers. I love her round-about way of talking that somehow always gets right to the point. One of those "fluffy" women who always see and understand more than anyone thinks.
Galahad Threepwood: Brother to Lord Emsworth in P. G. Wodehouse's Blandings Castle stories. Author of a scandalous manuscript of memoirs and quite a lively story-teller.
Sergeant Mullins in the Frances & Richard Lockridge series starring Pam & Jerry North. Mullins knows that the crime is going to be a "screwy" one whenever the Norths are involved.
Marion Halcombe in The Woman in White by Wilkie Collins. In my opinion the best character in the book. She's got spirit and is very independent.
Marvin the Paranoid Android in the Douglas Adams books. Afflicted with depression and boredom, partially because he "has a brain the size of a planet" that he rarely gets to use. You'd think someone that depressing wouldn't be a favorite--but I just find him hysterical.