As crime fiction scholar Kerstin Bergman explains in an essay on Swedish procedurals,
To be a police procedural, a novel must have a set of police characters and--preferably detailed--descriptions of their work as they investigate one or more crimes.... Police procedurals generally display a police 'team of individuals, separated by age, experience, gender, race and ethnicity, [who] work collectively to restore and maintain social order.' *Unlike other types of mystery such as the private detective novel or the cozy mystery, which tend to feature a single protagonist solving the crime with the help of one or two assistants, procedurals reflect the reality of actual police investigation in which a team of professionals, including homicide detectives, supervisory and management personnel, forensic specialists, and forensic pathologists or coroners come together to play their respective roles in the investigation of the case. In order to maintain a level of verisimilitude that readers of procedurals demand, the author must depict these various team members to some degree or other as the case unfolds.
As The Guide to United States Popular Culture further tells us,
The method for solving crimes also helps define the police procedural. The investigative process involves a set of established professional procedures for collecting evidence, interviewing witnesses, examining crime scenes, and conducting forensic inquiries.... The group dynamics are often highlighted, re-creating in fiction the friendships and hostilities that exist among any group of people that work together.**With these critical explanations of the sub-genre in mind, readers may better understand that they will encounter a roster of characters in the March and Walker series, rather than two protagonists working in relative isolation to close the case in super-hero fashion, and that the respective roles and various relationships of the characters will be clearly explained in the story.
As an Amazon Vine Voice reviewer stated about BURN COUNTRY, "Kevin Walker and Ellie March each go their own way in this case, as part of a larger investigative team. The interplay between the various characters feels genuine, with dialogue and exchanges that give us a good sense of each of them as well as their relationships with each other."
So whether it's Swedish police procedurals, American Ed McBain and his 87th Precinct, or the March and Walker Crime Novel series, readers who favour this sub-genre of mystery stories understand that a realistic cast of characters is all part of the police procedural experience.
*Kerstin Bergman, "The Well-Adjusted Cops of the New Millennium: Neo-Romantic Tendencies in the Swedish Police Procedural," Scandinavian Crime Fiction, Andrew Nestingen and Paula Arvas, eds. Cardiff: University of Wales Press, 2011, pp. 34-45.
**"Police Procedural." The Guide to United States Popular Culture, Ray B. Browne and Pat Browne, eds. Madison: The University of Wisconsin Press, 2001, pp. 617-18.