Friends, Neighbors, Bosses, Parents, Children and some times animals. There are things you need to know about these secondary characters.
A brief description, honing in on one particular habit of looks, speech or actions. These are the characters who show support for your main characters, or not. Usually not villains, the secondary character can take on this role for a reason. Her best friend may not thing he's right for her and she will certainly tell her. A child can take many roles in the story.
Some of the things you need to consider when developing a secondary character add to the interest. What is their relationship to the main character? What interactions do they have with the main character or characters? Have you given the main character too many secondary characters and thus muddy the water?
In one of my series, there is a secondary character who is present in all the stories. She's into Astrology and spends her time telling her friends who is and who is not right for them. Hers is the last book in the series since everyone wanted her to have her own romance. In another series some of the characters are seen or mentioned in the other stories and play a small or large part in the story development.
An important thing is to not let the secondary character or characters take over the story and become more interesting than the hero, heroine or villain. So develop your secondary characters as sidekicks to the main characters.
A word about using animals as secondary characters. They can sometimes take over the story and while we live our furry creatures alrge or small, they must remain in their place in a supporting role.