Minor characters in any story are characters the story does not center around. However, they often complete the main characters in some way and help move the conflict and resolution of the story forward.
One minor character in Cynthia Lord's novel Rules is Kristi, the girl who just moved in next door to the protagonist Catherine and is her age, 12 years old. Catherine is ecstatic to have someone her own age living next door because she feels she struggles to make friends due to her constant need to look over her young brother David, who is autistic. Kristi does eventually go over to Catherine's house to visit and, when David breaks out into a shrieking fit, tries to console Catherine by saying, "Even regular little brothers are a pain." However, the word "regular" helps escalate Catherine's internal conflict; she doesn't want to see either David or her friend Jason, whom she met at David's occupational therapist's office, as different from everyone else nor does she believe she should. By the end of the story, she accepts that there truly is no "regular." Also, by the end of the story, she is pleased to see that Kristi accepts Catherine's friendship with Jason.