Most stories have a score of minor characters, who shuffle in and out of the narrative. Usually, they have their "screen time", convey any information they have, move the story forward, and leave. Where do they go? What do they do next?
Most readers don't care and, probably, most writers don't either. After all, their minor characters. If we cared about what happens in their life (beyond how it impacts the main characters), we'd be reading their story.
But, sometimes, the writer stumbles across a minor character who refuses to quietly go away. Something about them sticks in the mind, compelling you to keep telling their story.
In those situations, the minor character sometimes ends up taking over the story. Or, at least, butting their way into a larger piece of the action.
In my novel, The Path Into Darkness, the character of Lucia was originally intended to be a minor, throwaway character. She existed, at first, only to show how inept the main character Mark was with women. Ultimately, though, Lucia was a bit too interesting of a character to just quietly go away. And, in this case, it was probably one of the better things I could have done with the story.
It ended up being a much stronger story once I "upgraded" Lucia from minor character to an "almost" main character. In doing so, you could see not only Mark's initial ineptness with women, but also some growth on his part due to interacting with her.
Do you have a minor character that refused to go away and ended up stealing some of the spotlight?