If you already have a cast of interesting characters in your series, why would you want to introduce another one? Consider this: writers insert new characters in a series when the protagonist solves the crime. The bad guy or gal is almost never someone who has appeared in the series before, although this could be done. In addition, suspects can be a mix of familiar characters and new ones. As for the victim? That’s usually someone the reader doesn’t know from the series, although it could be a familiar face. Sometimes a writer adds a new character, not a suspect or the victim. There are many kinds of new characters who each serve a distinctive function in the mystery.
Bad guys and gals
It has been suggested that the mystery is as much the bad guy or gal’s story as it is the protagonist’s. You may agree or not, but there’s a more compelling and interesting story to be told if the writer provides the reader with a character with depth, history and connections. A simple motive for the crime and a shallow development of the criminal cheats the reader of a challenging quest to solve the mystery. He or she can’t be all bad, but they can’t be all good either. Too much sympathy for the potential killer and the writer runs the risk of shocking the reader at the end when we really want to surprise and lead the reader to understand why that complicated individual committed the crime. And if our killer is too awful? The reader might come to suspect that person too early in the story or the reader may find such an unsympathetic character out of place in a cozy mystery where most characters are more nuanced.
Suspects are often characters the reader has not met before in the series, so they demand detailed attention. Introducing a character with a brief encounter and a clue to lead the reader to believe this one must be the killer is wholly unsatisfying and can read like what it is—a ploy to develop a list of suspects that the reader knows better than to consider as potential killers. These characters should also function as more than mere suspects. The writer must weave their behavior and personalities into the plot and provide the reader with reasons for why they appear in the work, i.e., are they relatives, friends, colleagues? They should have some relationship with the protagonist that is more than a brief appearance as the greeter at the big box store.
Victims are not only important to the mystery because it is their murder the protagonist must solve, but the protagonist must have a reason to get involved. The victim in a cozy mystery is usually someone the protagonist knows, but it’s often not a character familiar to the series reader. The victim is usually a new character. If the reader knows the character from previous books in the series, the writer confronts the issue of killing off a character a reader may like thus adding another level of complexity to both the protagonist’s motivation to solve the crime and a reader’s feelings about the loss of a perhaps well-liked character. If we assume the victim is a new character, the task of the writer is clear: give the protagonist reason to get involved. That means making the victim a character the protagonist cares about and so does the reader. Here the writer must be careful about overuse of backstory and allow the victim to be revealed in a series of events and dialogue. If the murder occurs in the first paragraph or on page one, the writer has some work to do in making the victim knowable to the reader.
A new character for unexpected twists
Sometimes it’s tempting to introduce a new character, who may become one of the suspects, but could appear because the writer wants to deepen the protagonist’s character in a manner not found in previous books in the series. The writer might also use a new character to brighten up the series. A series can become stale, and new blood can perk it up as well as introduce aspects of the protagonist’s past and personality the reader was not aware of. Developing new characters can be fun for both writer and reader. Few protagonists are iconic enough not to need continued development in a book or throughout the series. An encounter with an old boyfriend, a long lost relative or even an enemy from the past allows the writer to broaden and deepen the protagonist to give the reader new insight into her or him.
I’m sure you have examples of how you’ve added new characters in your work or have noted the ways writers introduce new characters. Maybe you know of other reasons for adding a character in a cozy mystery.