Anyway, in the episode last night, thriller writer Castle is trying to cheer up homicide Detective Beckett by reeling off a list of fictional detectives and asking her what they all had in common and she says "a plucky sidekick?" The answer he's going for is that they all made major breakthroughs after getting thrown off a case, but that's neither here nor there. What it does is lead into a running theme for the episode of whether Castle is Beckett's plucky sidekick or a partner.
She points out that the plucky sidekick always gets killed.
The Word Detective, one of my favorite etymologists, says here (you have to scroll down) that "sidekick" is a term that "first appeared in the slang of the criminal underworld about 1906, and originally meant a close confederate or accomplice in crime" and that
The specialized slang of pickpockets may supply some clues. To a pickpocket, a "kick" is a pair of trousers, and, more specifically, the trouser pockets. "Kick" in this "pocket" sense first appeared in the mid-1800s, and to this day "kick" is used as slang for a roll of bills or other stash of cash. The side pockets of a man's trousers have long been known, logically enough, as the "side-kicks," and it seems plausible that this term eventually came to be applied to a criminal confederate who was as close to the speaker as the pockets in his pants.
So, it's really the storytellers who picked up sidekick as a literary device. The slightly lesser-status assistant to the protagonist. Forever Tonto, never the Lone Ranger. Dramatically unfair.
I notice in my own stories that the sidekicks are nearly always animals. In OBSIDIAN, he's a cat named Darling, with a distinctly un-darling personality. I think this comes naturally to me. Cats have always been a part of my life, always nearby, weaving through my days. Like any good sidekick, they're always secondary characters in my personal drama. There for comfort, for comic relief, to show me where the best patches of sun are.
I bet they'd make excellent pickpocketing accomplices, too.