Your protagonist lives in a world populated, usually, by many millions of others. A few of these will be special acquaintances that either help or hinder his quest. A short list demonstrates what roles they play.
1) They reveal key details without the need for the dreaded data dump. Dialog is always more interesting, because it brings characters to life, and readers simply listen in on a conversation.
2) They motivate the hero and serve as his conscience, depending on how close they are. Drama and conflict result if they strongly disagree with the hero's course of action.
3)They help define the story world and setting. Your world will be full of different tongues and cultures. A supporting character can bring this to life, again, without the need for exposition.
4) They progress the story in some way. That means they should only be on stage when there's a reason for it. Readers are anxious to know what happens next, not the details of Bert's son's birthday party.
5)Friends or enemies contribute to the hero's own backstory. This means they'll need their own history, broadly painted in. Close friends reveal key details of the hero's early history, maybe something he's in denial about, and is causing him problems.
6)Limit their movement, unless he's a close companion vital to reaching a destination or who serves as a bodyguard, etc. That helps readers remember who they are.
If you find yourself unduly interested in a secondary, consider spinning him off to a series of his own. The illustrations show an example of this. In keeping with our Halloween theme, the second view shows a mage called Wicca in an early scene as a villain. The dwarf at upper left later becomes her sidekick, so it's fortunate her spell missed hitting him. The first illustration shows Wicca on modern Earth, where she's the hostess of a Saturday horror show.