The hero gets the arc. The sidekick gets the job.
That sounds like a demotion and it is the opposite of one. A protagonist alone on the page is a closed system: everything they notice, they notice silently, and everything they decide, they decide without argument. Put one other person beside them who is not an enemy and three things become possible that were not possible before. The protagonist can explain something without the author reaching in to explain it. They can be disagreed with out loud. And they can be seen, by someone whose opinion they have to live with.
That is a structural job, not a personality type, which is why the sidekick survives every change of genre and format. Swap the sword for a revolver, the castle for a spaceship, the novel for a comic strip, and you still need someone for the lead to talk to. Here are nine who do the job unusually well, across epic, satire, detective fiction, children's books and one newspaper strip.
1. Enkidu, in The Epic of Gilgamesh (c. 2100 BCE)
The oldest surviving one, and he arrives as a corrective. Gilgamesh is a tyrant nobody can check, so the gods build a man strong enough to fight him to a standstill, and the fight ends in friendship rather than a winner. Everything the story cares about happens after that: the king now has a peer, and a peer is the only thing that can tell a king he is wrong. When Enkidu dies, Gilgamesh does not become a better ruler, he becomes terrified, because the death is the first thing he cannot out-muscle. Four thousand years in, the structure is already complete. The companion exists to make the protagonist reachable.
2. Sancho Panza, in Don Quixote by Miguel de Cervantes (1605)
The most useful sidekick in the language, because he is the one who says what he sees. Quixote looks at a windmill and reports a giant; Sancho looks at a windmill and reports a windmill, and the gap between those two sentences is the entire novel. What makes it durable is that Cervantes refuses to make Sancho merely right. He is greedy, credulous in his own direction, and increasingly infected by his master's frame. A sidekick who is simply the voice of reason is a device. One who is being slowly changed by the argument is a character.
3. Dr. Watson, in the Sherlock Holmes stories by Arthur Conan Doyle (1887 onward)
Watson is the reason the Holmes method is legible, and the mechanism is worth naming precisely: he is not stupid, he is ordinary, and the deductions are explained to him because he asks the questions an intelligent person would actually ask. Modern adaptations that play him as a fool break the stories, because then the explanation is charity rather than conversation. He is also the narrator, which quietly solves the hardest problem in detective fiction: the reader cannot be inside the detective's head without the puzzle collapsing, so the story is told from a foot to the left.
4. Marian Halcombe, in The Woman in White by Wilkie Collins (1859)
The companion who is plainly better at this than the hero, written sixty years before anyone framed that as a joke. Walter Hartright is the nominal protagonist; Marian is the one who investigates, who keeps her nerve, and whose diary supplies the novel's central stretch. Collins gives her the intelligence and the courage and withholds only the social permission to act on either, which is what makes the arrangement land instead of reading as a modern retrofit. Worth studying for one more reason: the book ran as a weekly serial, and Marian is how Collins kept a reader oriented across nine months of instalments. She is the voice you re-enter the story through.
5. Jeeves, in the Jeeves and Wooster stories by P. G. Wodehouse (1915 onward)
The full inversion, and it is the funniest use of the archetype anyone has found. Bertie Wooster is the hero and the narrator and the idiot; Jeeves is the sidekick and vastly his superior in every measurable way. Wodehouse keeps the arrangement stable with one rule: Jeeves never takes over. He solves the problem sideways, accepts the credit obliquely, and extracts payment in the form of a disliked hat being quietly destroyed. Read as craft, it shows that the role is defined by position rather than competence. The sidekick is the one standing next to the story, not the one who is worse at things.
6. Piglet, in the Winnie-the-Pooh books by A. A. Milne (1926)
Children's fiction gets the archetype right more often than adult fiction does, because it has to do it in fewer words. Piglet is a Very Small Animal and is frightened of nearly everything, and the books never once treat that as a flaw to be cured. His function is to be the one who does the frightening thing anyway while admitting he is frightened, which is a far more useful model of courage than a character who is not scared. He also makes Pooh legible, in exactly Watson's way: someone has to be there for Pooh to think out loud at.
7. Hobbes, in Calvin and Hobbes by Bill Watterson (1985 to 1995)
A four-panel strip is the harshest possible test of this structure, and Hobbes passes it daily. He is the audience Calvin performs for, the only one who can puncture him, and the one relationship the strip refuses to resolve: whether he is a real tiger or a stuffed one is never answered, because answering it would break the job he does. The strip's emotional range exists entirely in the gap between how Calvin behaves alone and how he behaves with Hobbes. That is the sidekick function stated in its smallest possible form.
8. Mervyn Bunter, in the Lord Peter Wimsey novels by Dorothy L. Sayers (1923 onward)
Sayers took the detective's-companion role and made it professional. Bunter is Wimsey's valet, his forensic photographer, and the man who served under him in the war, and that last fact is doing quiet, enormous work. He has seen Wimsey at his worst and stayed, which means his loyalty is evidence rather than decoration. Where Watson is the reader's proxy, Bunter is the one person in the book qualified to worry about the hero, and the novels get their unexpected weight from letting him.
9. Samwell Tarly, in A Song of Ice and Fire by George R. R. Martin (1996 onward)
The modern epic-fantasy case, in a series that is unusually hostile to competence being rewarded. Sam is a coward by his own accounting and a scholar by disposition, in a story where neither is currency. What he does is remember things and write them down, and the series keeps demonstrating that this is the rarer skill. He is also the clearest recent example of a sidekick whose usefulness is knowledge the protagonist does not have, rather than support for a protagonist who already knows everything.
What the sidekick is actually for
Read together, the nine have almost nothing in common as people. They are ancient and modern, brave and frightened, servants and equals, more competent than the lead and much less. What they share is a position: close enough to be told the truth, far enough to have a different opinion about it.
If you are writing one, the test is not whether the character is likeable or funny. It is whether removing them would force you to move information into narration. If the answer is yes, they are doing the job. If the answer is no, you have written a second protagonist with less to do, and readers feel that even when they cannot name it.
The role earns even more in serialized fiction, for a reason that is purely practical. A reader who comes back to chapter nineteen after a week away has lost the shape of the story. A familiar companion is how you hand it back without a recap: they ask the question the reader has forgotten the answer to, and the protagonist answers it in scene, in character, with no author stepping in. The most reliable re-entry point in a long serial is not a summary paragraph. It is two people who already know each other, talking.
If you are writing that kind of story chapter by chapter, publish it where readers can follow it that way. Start a serial on Fictionate, or read what other writers are building there.