One of the things a writer can hardly avoid is characters. Almost all stories have at least one character, most have several. You have a story you want to tell, there are more characters in there than just the main character, so how do you keep the others interesting?
There are two kinds of characters. Viewpoint and (doh) non-Viewpoint. Which can be subdivided into sidekicks, supporting cast and extras.
The Viewpoint characters have their story told from their perception. The story enters their mind and the camera shows the story unfold through their eyes. Since George R.R. Martin‘s “A Song of Ice and Fire” series the multiple viewpoint books have risen in popularity, both by readers and writers. This is however an old construct that existed pre-Martin. Nexus (then called “ZaakX”) already had that structure, and many more books do.