Three types of characters: major, supporting and invisible characters

Major characters show up often in the story. Readers will care about them. Readers expect to find out what happens to them by the end of the book. Obviously, among your major characters, the first and most important one is your protagonist. No matter how many major characters you have, there should be only one protagonist.

Supporting characters.
Supporting characters may make a difference in the plot, but readers aren’t emotionally involved with them. They may cause a twist in the story, but they will not play a major role in shaping the story.
A supporting player does one or two things in the book and disappears. You may name supporting players and perhaps divulge a little bit of background, but they are not as important as major characters are.
Examples of a supporting character would be a man who discovers the body in a mystery, the crazy ex-girlfriend in a romance, or the guard of the Seat of Death in a science fiction book.