In my first published novel, Retirement Homes Are Murder, I originally planned to kill off one of these secondary characters. My editor, Deni Dietz, intelligently pointed out that I might want to keep this character around for future books in the series. She was right. Instead, he suffered a heart attack but survived for another day to keep up the banter with my protagonist. Readers liked that this character continued to be present in other books in the series.
Quirky characters sometimes have a life of their own. They appear and then start playing more of a role in the story. This is fine just as long as they don’t usurp the position of the protagonist.