Each and every character in a story has a single purpose for the storyteller - and that is their role or agency vis a vis the moral development of the protagonist, hero or heroine, the subject of your novel. They either assist or hinder.
There are filler characters, the door holders, and bag carriers, the petrol pump attendants, and these too may have something to add, but you need to concern yourself with the team first and foremost. Some 3-15 players.
Wherever you are in your novel, you should be looking at the crucible - the hell Sartre might have dubbed it or heaven - that is the nexus of relationships. What people want from your main player and what he or she wants from them. The relationships - the push and pull of their recriprocal wants and needs - will range from highly negative and reversing (antagonist) to assistance, affection, love and self-sacrifice (the friend or lover).
So you'll be playing these out in your story through conflict and clarification - questions posed...