Most protagonists are not complete loners. Interactions with a sidekick, best buddy or groups of friends typically make up a good portion of the story. Creating the peers of the main character may actually take more thought than creating the main character. Many authors rely on their own experiences to flesh out the major person in the story. However, they have to guess what is inside the head of the companions and must stretch themselves beyond the comfortable realm of “write about what you know.”
However, observation of real life helps when creating an interesting interplay between the main character and friends. A peek inside of what helps real groups to solve problems may be the key to creating the cohort that complements the unforgettable protagonist. The current emphasis on collaboration would lead people to assume assembling a large enough group could solve almost any problem. You really don’t believe this do you?
Create a group of more than five people in fiction and readers have difficulty remembering who’s who. This is not much different from real life. In large groups people stop contributing and start a tactic known as social loafing. Research has shown that somewhere from 4 to 6 is the magic number for a truly collaborative group . Any size beyond that and people will follow the loudest voice that supports the status quo . Maintaining status quo not only limits creativity; it also makes a boring plot line.
Unless you are writing the epic novel of your era, too large a number of well-drawn auxiliary characters dilutes the attention that your reader will invest in the struggles and triumphs of the main character. I used to keep lists of characters when reading epic novels, but then I enjoyed studying literature. I am not your typically reader.
In real situations, diverse groups are smarter and better at problem solving . So add someone of the opposite gender as a friend. Put in a best buddy from another city, country, continent or even another world. Different ethnicity makes different viewpoints more acceptable because, oddly enough, people become more upset when a view that conflicts with theirs is proposed by someone that looks and acts like them, rather than someone who is obviously different.
This kind of attitude occurs in the fictional world also. Consider the difference between a foil and a doppelganger. Cervantes’ Don Quixote is tall, thin, and an impractical idealist, while his sidekick Sancho Panza, is short, fat and a realist. These differences in temperaments and appearance not only play the main character off of his side kick, but provide a range of resources so that they can assist in helping each other get out of trouble.
However, the when the main character has a twin, a person who appears the same physically but is on the other side of the behavioral spectrum, the outcome is virtually bound to be tragic. It doesn’t matter whether the twin is an actual human, as in the Charles Dickens’s Tale of Two Cities, a mythical doppelganger, as in Edgar Allen Poe’s William Wilson, or simply an illusion as in Fyodor Dostoevsky’s The Double. One (or sometimes both) of the identical appearing people are almost always doomed. There is an occasional exception. In Joseph Conrad’s The Secret Sharer the young captain benefits from saving the life of his criminal look alike.
When it comes to creating the friends of the protagonist, you may not be able to always write what you know, but look at what others have written, both in fiction and real life research.
 Woolley, Anita. and Malone, Thomas. Defend Your Research: What Makes a Team Smarter? More Women, Harvard Business Review Magazine June, 2011.