Secondary characters are important in a story- any story. With them they help move the story along, they help your main character which can be good or bad and they make your story well rounded. They help reveal details, can motivate the main character and interact with the antagonist even if it is them.
A lot of people think it is all about the antagonist and protagonist. But we can’t be remiss about the other guys. The ones who are basically side kicks to our superheros of the novel.
Treating them like well second class citizens does them, your main character and your book a disservice.
I have read books where I preferred the secondary character so much more than anyone else. Just like a tv show or a movie where you prefer the supporting cast much more.
They help open up dynamics to your story line and possibilities that you would not get with just a single character. Unless you are writing a book where a person is all by themselves
Don’t let your book down by ignoring this important part of a novel. By doing so your book will be one dimensional. And unless you are writing a one character book your novel will have a strong shot at going nowhere.