In case you don't know what a recurring character is, the idea is simple. These are characters who show up from time to time in a story, but not so often that they're considered one of the main/supporting characters.
In a TV show, this might be a character that shows up in a few episodes each season, but that aren't in a majority of the episodes. In a book series, it would be a character that's in multiple books in the series, but not all of them (think Michael or the Alphas in Dresden Files). In an RPG, this could be an NPC that the party sees several times, but that doesn't show up every session/adventure.
Why Use Recurring Characters?
Put simply, audiences, be they readers, viewers, or players, like familiarity. They like when a character walks onto the screen and they know what they can expect from that character.
Because they aren't part of the main cast, sometimes recurring characters can be more dramatic or shake things up more than people who are always present can (since those who are always present generally have to maintain similar relationships to everyone else, or primarily make choices that can later be readjusted back). This allows these recurring characters to sometimes leave more of an impact on the audience, gaining a particular following and favoritism that gets audience members excited when they know that character will be returning.
It's good to use recurring characters when possible. Rather than introducing a new character and adding to a growing cast, putting in a familiar face helps the audience to connect by utilizing an already established connection.
Examples of Recurring Characters
1. The Old Friend
This is someone from the past of one of the main characters. One of the more common/effective uses of this character is to have them be a former best (or at least close) friend of one of the main characters. They may still consider themselves to be friends, but they certainly aren't as close as they once were.
One of the benefits of the old friend is that it allows the rest of the main cast (as well as, or even more so, the audience) to learn more about the regular character's past. Possibly, if the regular character talked or acted differently in the past, the old friend being around may even cause them to revert back and pick up some of their old habits.
In addition to this, drama can be ramped up in a few of ways. The regular character who is friends with the old friend may care a lot about getting the rest of the main cast to like the old friend. Dialed up if the old friend is expressly the sort of person the main cast doesn't get along with, or vice versa. The old friend may be jealous of the regular character's friendship with the rest of the main cast, or vice versa. Further, the old friend may have changed from the person the regular character remembers. This could create tension as the change makes the two less compatible, or if the old friend tries to hide this change. It can also be played the other way, with the regular character being the one who has changed, and this can even be used to show character growth. Another way the recurring character might hide things about themselves is if they are just trying to use their friendship with the regular character for some advantage that they are otherwise keeping secret: money, a place to stay, access to the regular character's work or friends, etc...
This is more commonly used in sit coms, but it could be worked into RPG scenarios or other works as well. In an RPG, having people that one of the PC's grew up with show up is always interesting. Maybe they lived in the same village as kids. Maybe they trained at the same academy or went to the same school. Maybe the old friend is a member of the PC's former adventuring company.
2. The Needy Relative
A person connected to one of the main characters shows up. They need something. They may or may not be open with the main character about that, but their presence will be clear to the audience as a metaphorical weight around that character's neck.
There may or may not be some overlap with this and the old friend, especially as many relatives tend to have ties to a character's past. Relatives also have ties to a character's present by way of the family, and can be a way of showing the people a character is connected to in that way. If other family members have shown up previously, a relative showing up can provide an update on the status of something going on with that family member, helping to establish more familiarity with the audience and providing them more information on a situation they might care about without having to do a whole episode/scene/chapter/lengthy thing about it.
Drama with the needy relative usually comes from the need. Like the old friend, they may be concealing this need and plan to take advantage of the main character covertly. Possibly they are a pickpocket who is fleecing the main cast for every dime they can get, or possibly they are less sinisterly just trying to use their connections to gain an advantage-maybe a job or access to someone for which they have feelings. They may, however, be open about their need, relying on the character's sense of duty to their family or general righteousness to gain their help. Possibly, it might be a health issue or something else for which the expression of the need isn't desired by the relative, but that it is necessary.
This is common in dramas, sit-coms, and the like, but it can be used in any story. In an RPG, nothing motivates a PC so well as someone they already like and have connections to asking them for help.
3. The Love Interest
Used in stories where the love interest isn't also one of the main characters, this is someone that one of the main characters has affections for. If it's a recurring character, it's often a plot that advances in phases over time, but sometimes not.
Usually, the main character will develop affections for the love interest, and there will be a pre-relationship phase where the audience is teased with the will-they/won't-they drama. If the love interest doesn't reciprocate the main character's feelings, then they usually don't stay in the story for very long after, making them less likely to be a recurring character.
Once the couple enters the relationship phase, the character is free to come and go in the story as a tie to that main character as needed and for any number of uses. This can reveal things about the main character and how they treat relationships, but a love interest can also be used as an extra main character for stories where an extra person in the group would be helpful.
In RPGs, many groups don't like to utilize romantic plots due to awkwardness of the meta-issues of a DM and a player playing characters that have feelings for each other, especially if there are romantic relationships going on in real life between any of the involved parties. That said, for the groups that do utilize romantic plots, it can be a strong motivator for PCs. A PC will go to great lengths to win the affections of a love interest, and once having them, they will often do many things for that love interest to support the relationship and show their love. Love interests endangered motivate PCs, but this should not be done so often that it makes PCs afraid to develop connections to people for fear of being punished by their loved ones always being in danger.
4. The Ex
Maybe the main character had broken up with their ex at a point before when the stories began, or maybe she was once a love interest who moved down to this position, having a former flame show up in a main character's life is a sure way to generate drama.
The ex can be utilized in a number of ways, depending on how both the ex and the main character they are attached to feel about each other at present. If one wants the other back and the other doesn't, there are a series of issues that come with how they handle that. It could potentially even show character growth in a main character who has moved on. If they both have resentment towards each other, the return of the ex can be antagonistic to both sides. If they both want to get back together, then either the ex might move up to being the love interest, or they might be reminded of why they never worked out in the first place.
3. The Co-Worker
This is someone that one of the main characters works with. The main character usually either likes this co-worker, or they don't, but they're rarely undecided.
If it's someone the main character likes, then they may have a similar effect of the needy relative, using this affection to try to get the main character to help them with work projects to boost their own success. If a friendship develops with the co-worker, they might have some similarities to the old friend, with tensions being caused if the co-worker doesn't get along with the rest of the main cast, or if there is jealousy on either side. It may become a workplace romance, going into the territory of the love interest, but adding any tensions of having to work together on top of their romance.
If it's someone the main character doesn't like, the issues caused can range from annoyance (see Patrice from How I Met Your Mother) to active sabotage from either side. If this co-worker is in the same position as the main character and they are both competing for the same projects/jobs/promotions, it may extend into the territory of the rival.
4. The Business Contact
This is someone who a main character has to work with for their job, but who they don't see regularly. In spy fiction, it may be a contact on the ground in the location. For someone who works in an office job, it might be a supplier or a potential client for their business. Usually, these contacts don't work/live near the main character and they show up either if the main character goes to their location, or if they come to visit where the main character works.
5. The Staff Member
This person works somewhere. Somewhere the main cast goes often. Maybe it's a bartender that shows up from time to time. If the characters are accident prone or have health issues, maybe it's a doctor (one of my favorite recurring characters is the doctor in Arrested Development). Especially in RPG games, it could be a quirky and interesting shopkeeper.
These rarely increase the drama or plot of the story, but they do help to provide familiarity. If they have defining/interesting personalities, they can also provide humor or fascination for the audience.
6. The Villain
This is the bread and butter of comic book superhero stories. This is the Joker, Loki, Dr. Doom, the Riddler, Green Goblin, Lex Luther, etc...
These are the characters who show up over and over again and ruin the main character's lives. They pose a threat that forces the main character to act or lose something important. When they come onto the scene, they come with all the existing tensions of the previous times that they've fought against the heroes.
Audiences like recurring characters, but they love recurring villains.
For me, I think that a lot of times I find myself rooting for the villain even more than the hero, and I don't think I'm the only one who does. Something about villains fascinates us. We want to know how they think, how they do what it is that they do. Add to that the familiarity of seeing them over and over again, and some additional quirks and attributes that charm an audience, and you've got a recipe for something audiences can't turn down.
In addition to all that, we as people process most things through conflicts. Through overcoming adversity. This is what the majority of stories are about. Seeing the hero in conflict with a villain is the story conflict going back through the ages. It's familiar. It speaks to us on an innate level. Having a recurring villain of great power magnifies that over and over again.
The biggest issue with recurring villains, especially in RPG campaigns, is how to keep the heroes from killing them. In stories, it's easy, because the creator controls both sides of the board. The heroes have qualms about killing people, or the villain has an escape route and gets away, or the hero puts the villain in prison (only to have them escape later on. Seriously, does Arkham just have a revolving door in the back of it?), but one way or another, it's easy in fiction to make the villain able to come back later on.
In RPGs, where PCs are often murder-hobos, it's a little bit harder. If they enter combat with the villain, they're often going to kill said villain, making it harder to make them recur. Here are some suggestions for how to deal with that: You could have the villain interact with them in settings where they can't kill him. Maybe he's a well respected noble and they meet him at a party where no one would believe them if they accused him or tried to battle him. Maybe he's talking to them from atop a cliff or otherwise in a place they can't reach. You could have the villain interact through minions or messages, which makes their presence recurring, but it doesn't seem to actually have the villain itself be recurring. You could use things like holographic projections or robots that look like the villain which are directly controlled by the villain. You could make sure the villain has an escape route whenever the PCs would encounter them. (This should be done carefully, as PCs are clever and may find a way to cut off this escape route, or, especially if the villain tries to fight before taking the escape route, they may kill the villain before he/she can escape. The only way to be sure of an escape would be to fudge die rolls or how much damage the villain is taking, which some people may see as underhanded/unfair GMing, even if it is for a better story.)
One way of having a villain recur, either in stories or RPGs, that I am wary of, is bringing them back from the dead. I'm not saying that this can't be done effectively, because it can. I've seen great examples of it that I've really liked. However, if your work, be it an RPG, or a book, or a tv show, or anything else, brings people back from the dead too much, death will become meaningless. Your audience will stop caring when people die, which takes away the emotional consequences of death in your story.
7. The Rival
One might consider the rival a subset of the villain, but it is a subset that is specific enough that I wanted to talk about it.
The rival is a great recurring character. It is someone who reminds the audience of one of the main characters, because the share many of the same attributes. They're good at the same things. Oftentimes, if the story is of a sort where things like darkness or evil make sense, the rival might be seen as a dark/evil opposite of the hero, using the same skills and attributes in more sinister ways.
But one of the things that makes the rival particularly compelling is that they usually want the same thing the main character wants, and that thing doesn't split between them. Maybe it's a promotion at work. Maybe it's the affections of a particular love interest. Maybe it's a lost relic of an ancient civilization. Maybe it's a super weapon.
The rival is great because it raises for the audience the tensions of a race to the finish line as well as questions about which side's skills and tactics will prove superior. Since usually (but not always) the rival is more willing to be underhanded than the main character, they have something of a leg up from the start.
What other recurring characters have you found that you've liked? What do you like about them?