Many people lead themselves to believe that the protagonist (hero) and antagonist (villain) are the only important parts of a story. They're what make up the action, right? But this isn't the case. Not in the least.
There are so many more characters important to a story than just the "main" ones. Let's take the example of Harry Potter. Besides the "trio" (Harry, Ron & Hermione) there are tons of characters that are minor (though "Potterheads" may disagree to that term) such as Dumbledore, Dobby, Neville Longbottom, Luna Lovegood... All of these characters are perfect examples of minor characters that are absolutely essential to the progression of the books. Alternately, beyond Lord Voldemort, there are essential characters on the opposing side: Draco & Lucius Malfoy, Bellatrix Lestrange, Wormtail.
In my mind, all of these characters should have had just as much thought put into them as the main characters. Every character that is involved in the evolution and progression of a storyline should be well thought out and three-dimensional (unless intentionally made to be otherwise).
In my current project (currently unnamed) the main protagonist is a young werewolf named Sarina and the main antagonist is an ancient vampire named Elliot. They're locked in a seemingly never ending dance of death, each trying to eliminate the other. However, the evolution of Elliot throughout the story is dependent solely on Rikki, a girl he meets some decades after his first encounter with Sarina. Oppositely, Sarina's courage and skill, as well as her drive to kill Elliot is led on by her human friend, Ethan.
While Elliot and Sarina are the key points of the story, Rikki and Ethan deserve as much thought, because they are so essential to the plot line.
There are a few things to keep in mind when you're deciding the most important characters in a project:
How Often Are the Characters Around?
Nine out of ten times, if a character is a constant within a story, that character is important. Even if they aren't important, they should be well-planned. A flat character that's always around is no good.
Examples of this are (going along with the Harry Potter example) characters that were in the same house as the trio, such as Seamus Finnigan. Being around as frequently as he was throughout the series, it was important that he was well-developed and had a definite personality.
Does the Character Have an Effect on a Main Character?
Any character that has, will or is having an effect on the main character(s) of a story are important. Characters that are parents/guardians or leaders need to be well-defined, because their personality has an effect on how your character(s) react to them.
An example of this is Dumbledore. Though he himself is not a main character, he has a huge impact on Harry's life. If he were anything other than what he was, things with Harry might not have ended up the way that they did.
Does the Character Have the Potential to Become Important?
This is a very vague question, but should still be considered. At the point you're at in your story, you may have plans to make a character important. But that doesn't mean that they can't. What you have to ask yourself is, "Is there a good chance that this character can have a larger role to play?" This is only a question that you need to ask yourself i you have very little of the book planned and you have a large group of characters prepared.
Developing your main characters is hugely important, but developing the characters around them is almost as important. A story is flat if its characters are, and that includes the ones that your main characters interact with.
Hope you enjoyed and found this post helpful!