As a show on a small budget, Forever Knight could not afford to have a large regular cast—that is to say, a large number of actors who appeared in every episode.  There were three characters who appeared in all seasons:  the hero, Det. Nick Knight; his friend, Dr. Natalie Lambert, who seeks to return him to mortality; and his master, LaCroix, who wishes him to accept the life of a vampire.  However, although Nick and Natalie were in every episode of the actual series, the frequency of LaCroix’s appearances varied, increasing with each season.  In my own Season IV, all three characters are in every episode.
Since Nick is a detective in the Metropolitan Police in Toronto, two of the other regular character slots are given to his partner and his boss.  In Seasons I and II, his partner is Det. Don Schanke, who dies in a plane crash in the third season premiere (but makes a guest appearance in one episode of my Season IV).   The character of Schanke was replaced in Season III by a new partner for Nick, Det. Tracy Vetter.  The police captain in charge of the precinct to which Nick is assigned differed each season.  In Season III, it is Capt. Joe Reese.  Given that the episodes are usually dominated by a murder mystery plot, it is not surprising that these characters appeared in every episode of the season; and this is also true in Season IV.
Besides these, each season had another regular vampire character.  In the first two seasons, this is Janette:  as another “child” of LaCroix’s, she often appears in the flashbacks that recount Nick’s history down the centuries; but she also owns the Raven, a nightclub in Toronto.  For the third season, however, she was written out of the show and her nightclub transferred to LaCroix.  She appeared in only one more episode, “The Human Factor”.  Janette is reintroduced to the series as a recurring character in the third episode of my Season IV, “Crimson Joy”, though her nightclub remains the property of LaCroix.
In Season III, the actress who played Janette left the show; and the role of Third Vampire was instead taken by Javier Vachon.  The transient passage of this 500-year old vampire through Toronto is interrupted when he meets Tracy, who learns his true nature (though not that her partner is also a vampire).  Since this puts her in danger, Nick insists that Vachon remain in Toronto to protect her.  However, although the actor was credited in the opening sequence through the first half of Season III, the character actually appeared in fewer than half of the episodes.  Given the potential for comparisons between his relationship with Tracy and that of Nick with Natalie, I decided to increase the frequency of his appearances:  Vachon is in every episode of Season IV.

Although other vampires besides LaCroix’s family appeared in Seasons I and II, they did so as guests in individual episodes.  The only recurring vampire character was his daughter, Divia, who appeared in both “A More Permanent Hell” and “Ashes to Ashes”.  She also appears in one of the episodes of Season IV.
In the third season, though, the new character of Vachon was accompanied by the occasional appearance of friends, Screed, Urs, and Bourbon.  The first, with a picturesque taste for rat’s blood, was killed off mid-season in the episode “Fever”; but appears as a recurring character in the flashbacks of some of the episodes in Season IV.  Urs—whose job as a dancer at the Raven affords insight into the life of LaCroix as well as Vachon—has become a regular character, appearing in almost all episodes.  Bourbon, as the character in Vachon’s “crew” who was least well developed in the actual series, remains no more than a recurring character.

Forever Knight never had the budget for large numbers of those minor supporting parts that help to flesh out the “world” of a television series by filling it with familiar faces.  It is true that the character of Grace Balthazar, Dr. Lambert’s lab assistant, appeared in several episodes in Seasons I and II; the same news anchor appeared in two episodes in Season I; and the Raven had a recurring bartender, Miklos, and an unnamed waitress, who appeared in a handful of episodes in Season II.  But that was all; and even that was brief.  There were no such characters in Season III.
Since I decided to work on a slightly larger (albeit fictional) budget for Season IV, I decided to include a small number of supporting characters, though I use them only sporadically. I decided not to use any of the characters from Season II on the grounds that none had appeared in Season III, on which I was basing my own virtual season.  Instead, I came up with new characters, as needed.  It does mean, though, that familiar faces recur from episode to episode:    the desk sergeant at the 96th Precinct police station, a news anchor, and staff and patrons of the Raven.

For the most part, the guest characters specific to the various episodes of Season IV are new ones.  However, there is one particular exception.  In the Season I episode, “For I Have Sinned”, there is a priest at St. John’s Church, Father Pierre Rochefort, who plays a major role in the story.  This is reprised in the Season IV episode, “Confession is Good for the Soul”, when there is another series of killings.  In addition, Father Sean O’Connell, an older priest at St. John’s Church, appears in two episodes of Season IV.  This character may be identified with the older priest in “For I Have Sinned”, who was not named.  Unlike Fr Rochefort, Fr O’Connell should be considered a recurring character.
I have also expanded the roster of Tracy’s relatives.  Both her parents were mentioned throughout Season III, in which she also referred to at least one uncle and some cousins, though only one member of her family—her mother, Barbara—appeared in person (in “Fallen Idol”).  In Season IV, I treat Barbara Vetter as a recurring character.  Other relatives are mentioned by name; and several appear in the episode, “Broken Hearts”, particularly Tracy’s Aunt Rena.
As in the actual show, Tracy’s oft-mentioned father is never actually seen.