I don't wish to be a "Moaning Myrtle" 😁 but sometimes the secondary characters steal the scene and sometimes a majority of the book.
I have several strong secondary characters in my latest story, such that on a couple of professional edits the editors said that my main character was weaker that one of my secondary characters. And I had to agree, but it was just in this book. The main series of books, or the planned series, the main character does lead.
I have found in many of the books I have enjoyed the most they have many strong characters, some stronger than the main, though that does not detract, contrarily enhances the whole.
I am deep into Terry Pratchett currently and his characters, all, resonate with a personality such that you are pleased when they appear in the next story.
At the moment, I let my characters develop themselves as the story evolves, some have a very short lifespan, literally. But I don't know this until it happens. I like to be just a surprised as the reader. I may find this to be flaw later in the story and have to do some serious editing but as I am still a novice at story writing I have room to evolve my characters as my writing evolves.