I divide Secondary Characters (SCs) into three buckets.
Bucket #1: SCs who help humanize the MC. These include friends/allies, love interests, innocents who need to be rescued, and the comic.
(1) The Friend/Ally. This is someone closely associated with the MC (or the antagonist) and has some of the same virtues and values. For example, in the Hunger Games, Gale serves the role of ally for Katniss in District 12. At the level of plot, he’s her backup and helper, and her surrogate caretaker for Prim when Katniss leaves for the games. But at the level of character, he provides context. That is, Katniss’s ruthless streak might not be so acceptable to us except that Gale has it too—and because of this we understand that Katniss’s rough edge has evolved as a result of the harshness of her environment. Thus, our sympathy flows to her more readily.
(2) The Love Interest. Sometimes the love interest is an ally. But the love interest also provides an opportunity to show the vulnerability of the MC, who has a desire that may or may not be fulfilled.
(3) The Innocent. This SC is rescued by the MC. An example is Katniss’s sister Prim, who enables us to see Katniss’s deep loyalty and capacity for self-sacrifice, which makes her a compelling, admirable heroine. Significantly, the Innocent usually has a significant emotional tie to the Main Character prior to the first page. Another example is Dobby the House Elf, who needs rescuing by Harry Potter.
(4) The Comic. Ron Weasley and his brothers provide this in the Harry Potter books. Also Hagrid, with his unwise choices in raising dragons. Some other examples of comic relief are R2D2 and C3PO in the Star Wars movies. This SC can provide a respite in tension, without derailing the plot arc of MC’s intense desire. Also, when the MC laughs along, it shows his/her humanity.
Bucket #2: SCs who create conflict, confusion, or a sense of danger lurking. These include the foil, the could-have-been, the thorn, the foe (antagonist), the enigma, the betrayer, and the dead body.
(5) The Foil. This SC is often used to illuminate the traits of the MC. (“Foil” comes from the old practice of enhancing the beauty of a gem by placing it on a foil surface.) For example, in Anne of Green Gables the unimaginative but very beautiful Diana is a foil for imaginative, red-haired Anne. A foil can serve to represent an alternative value system. Peeta, the gentle baker’s boy, in The Hunger Games trilogy is an ally for Katniss; but he also serves as a foil for the more combative Katniss and Gale. A special subset of this SC is the Resistant Narrator who sees things differently than the MC—for example, Dr. Watson in the Sherlock Holmes tales. Sherlock is the MC, but Watson writes all the tales, with a very distinctive voice, and while they have the same goal (to solve a crime), Watson holds different values.
(6) The Could-Have-Been. These SCs present other options and can keep the plot from feeling inevitable; as such, they can shore up suspense. These characters can also show that a MC isn’t going to get everything she wants. (That’s a danger for books … when a MC can have her cake and eat it too.) A subcategory of the Could-Have-Been is the Fateful Warning—what the MC might be or become if circumstances were different, or if a character had made a different choice. In Les Miserables, Fantine is who Cosette might have become, if Jean ValJean hadn’t intervened. Draco Malfoy is a Fateful Warning (as well as a Thorn, see below) for Harry Potter.
(7) The Thorn. This character is bothersome, a thorn in the MC’s side but not the Foe (Antagonist). So for Harry Potter, the Thorn is Draco, not Voldemort. The Thorn can stymie the MC at various points and can contribute to the main struggle between the MC and the Foe. The Thorn can also be someone who recalls a past that the MC wants to forget, or who knows a secret about the MC that he doesn’t want revealed. The Thorn character can be the annoyingly nosy neighbor who offers all kinds of outlandish theories and red herrings to the detective; it can be the newspaperman who prints a story that unwittingly alerts a criminal to the movements of Scotland Yard. Thorns create annoyance and frustration in us and therefore help to engage our emotional energies and promote sympathy with the MC.
(8) The Foe (the Antagonist). This is the villain with a goal in direct opposition to the MC.
(9) The Enigma. This SC represents a puzzle and a source of anxiety because the MC can’t tell if he’s friend or foe, or what his motives are. Sometimes they seem to change from day to day. For Harry Potter, this is Snape. This creates suspense for the MC and the reader.
(10) The Betrayer. This SC is one of the heavy hitters. He betrays the MC in a way that pulls the rug out from under her, causes the MC to distrust her own judgment, or throws into jeopardy our interpretations of all that we’ve read so far. In One Flew over the Cuckoo’s Nest, Bibbitt betrays McMurphy to Nurse Ratched, who has longed to punish McMurphy, and she can finally justify giving him the lobotomy. In Harry Potter, this is Cho, who betrays Harry and his friends to Dolores Umbridge. In the third book of The Hunger Games trilogy, this is President Alma Coin, who claims to want democracy but turns out to be as power-hungry as Snow at the end. The betrayer is a very powerful SC, for betrayal can bring up feelings in your MC of injustice, abandonment, and loss—three of the big ones.
(11) The Dead Body (aka “the red shirt”) Often the dead body has a significant backstory because it contains the reason why he ends up dead. I put this SC last in the red bucket, but if you are writing a murder mystery, the dead body is arguably the most important SC.
Bucket #3: SCs who will provide information, expertise, or guidance to the MC. These include the Expert/the Guide, the Provider, the Mentor/Conscience, and the Plain Talker.
(12) The Expert. This knowledgeable SC provides expertise or information that the MC needs to achieve his goals. This can be as simple as Hermione Granger assisting Harry Potter with her superior book-knowledge. For amateur sleuths, this can be someone in the crime-fighting professional realm such as a Medical Examiner, a scientist, or a hacker. For a professional detective, it might be a ballerina who can give him insight into her cut-throat dance world. A special instant of the Expert is the Guide. This SC is one or two steps ahead on a learning curve. For example, if your MC has just lost her brother to cancer, the Guide perhaps lost his sister to cancer two years ago. The Guide can provide sympathy, understanding, or another perspective.
(13) The Provider. This SC is often a friend/ally, but his purpose is to offer an experience outside the MC’s normal life. For example, Harry Potter has no loving family who are alive; Ron Weasley has heaps of it, all boisterous and warm.
(14) The Mentor/Conscience. This is someone who provides guidance and/or shores up the conscience of the protagonist—often by providing insight or confronting the MC when he steers down the wrong path. This SC may be older or younger than the MC, someone who benevolently provides hard-won wisdom (Dumbledore or Professor McGonagall for Harry Potter) or with humane naivete (Scout for the community in To Kill a Mockingbird). But this SC can also have an adversarial relationship with the MC. In The Big Easy, the MC is a charming but corrupt cop in New Orleans, played by Dennis Quaid; Ellen Barkin’s character, a deeply ethical DA, works to expose the corruption in his department.
(15) The Plain Talker. This SC says things that the protagonist can’t. In the series The Crown, Princess Margaret is sometimes given this role; she blurts out truths that, for reasons of propriety and politics, her sister Queen Elizabeth can’t. This character can serve a variety of purposes. For example, if he says snide things under his breath about an unpopular character, he can demonstrate the MC’s tact or forbearance. Or he might say things under his or her breath that are truthful, or wryly observant; and by contrast, the MC can come across as naive or self-deceiving.
One final NOTE: If you give someone a name, they need a role, even if just a small one. Don’t name characters if they’re not going to do anything that either advances the plot or illuminates another character. The barista at the coffee shop can be elided: like the servants sometimes in Austen, “the tea was brought in.”
So those are my 15. Of course, these distinctions are sort of artificial. As this list suggests, often a Secondary Character will play more than one role—and generally that’s efficient so far as plot goes, and it organically adds complexity to your characters. It finds it natural echo in real life, for none of us plays only one role in our lives either.