In the world of secondary characters, from my researching – I have learned that there two types of secondary characters: minor characters and supporting characters.
Minor characters do not further the plot, and a story can essentially be written without minor characters. Minor characters are there to make the world in your story, real, essentially. Daily life would be weird without people filling the coffee shops wouldn’t it? So in essence, each minor character usually appears only once in the story – as a desk clerk maybe or a cab driver – then they are gone from the story forever. They are a necessity to filling up the world/story you created. But just because they are minor, doesn’t mean they can not help reveal certain characteristics of your main characters. Maybe you have a minor character that is an attractive man and your main character thinks hey, i should ask him out, but then is to shy to do so and just awkwardly asks for what she needs – this small scene right here just revealed an interesting trait of your main character. Keep that in mind when you are working with minor characters.
Supporting characters are more important and vital to the story than minor characters are. They appear more than once in the story because they are vital to the plot – they move the story forward in some way. These guys have a great impact on the story line even while maintaining the role as a secondary character. A great supporting/secondary character is Luna Lovegood from the Harry Potter series. She adds quirkiness and uniqueness to the series (even though the series itself is unique) which is great for a secondary character to do.