The 'Cast' is the thing...

...in which to catch the conscience of the 'King' ( aka the 'reader' of your book).

No good story is based entirely on one character. The main character usually has to play off or include others to resolve the storyline/ plot. Like in a good screenplay therefore , the series writer generally must assemble an ensemble cast of sorts to populate the content of the work and drive the plot(s) onward.

When one sees 'casting credit ' in a movie , one tends to think after the fact , what an easy job it was to assign this role to that person who it was clearly made for it . But... initially , no casting director or movie director can be absolutely sure the chosen one will suit and often only the very good actors can step up and make the role theirs.

Same it is in writing a novel/ mystery story. For example , although having a general idea of who I wanted in the defining roles , I wrote the first 2 storylines casting totally different characters (age, temperment etc) in the starring roles as well as the secondary characters. When I wrote the 3rd book, things gelled finally as a distillation of the earlier efforts and I had my cast of 'superhoroes' that I would be able to draw on individually, in pairs, as a groups etc. going forward. Consequently, these characters after being first introduced, became ever more defined and the writing becomes likewise easier knowing now how these characters evolved/are evolving , interrelating , and reacting within the plots.

So, if you intend to follow up with a Sequel or two or three... start rounding up and concentrate on developing your ideal cast. AND, along those lines , you might want to enter a contest that could convert your story and characters to a screenplay e.g. https://screencraft.org/shortstory/