Another aspect of designing characters has to do with the role each will play in your story.
A Review of the Basics
Primary Roles: the protagonist and antagonist
Secondary Roles: the sidekick, the confidant/mentor, the love interest
Minor Roles: bit players, people in the background, temporary characters, cabbies, messengers, etc.
Role helps define a character because readers bring certain expectations of what this individual will be doing. That in turn dictates some of the traits or attributes the character will have. From this framework you can then branch out.
Clearly the two primary characters will be the best designed among your cast. An entire story can be built upon these two people. They — along with their opposing goals — drive the story from its beginning to the final showdown and conclusion.
The secondary roles add complications and texture. They can enrich your drama, bring poignancy or comedic relief, and enable you to write scenes that aren’t always focused on the central antagonist. Both protagonist and antagonist can have their own sets of secondary characters to assist them as necessary.
Remember also that secondary characters can serve multiple functions. While you can give your protagonist two sidekicks, a mentor, and a love interest, thus creating four secondary characters to fill those roles, you can also combine them as needed. A single character may serve all three roles. Other combinations are possible in larger, more complex stories. For example, in the STAR WARS original trilogy, Darth Vader is Luke’s antagonist. He’s also the Emperor’s sidekick. Eventually, his dual roles collide, bringing him into a sacrificial choice at the climax.
The minor roles fill in wherever your story needs people. They add veracity and make the backdrop to your story events appear more plausible. They can be a prop or feature in a scene. It’s all up to what you require them to do for you.