I don’t think anyone likes reading a book or watching a tv show when suddenly a full-blown character, apparently with a fair bit of history with the other characters, shows up like s/he’s always been there. This isn’t a case of having missed his or her introduction in an earlier book in a series or a long-ago period of a long-running tv show. It’s a case of lazy, bad writing.
One of the numerous issues I had with Five Little Peppers Grown Up (1892) was the character of Charlotte Chatterton, the great-niece of the now-deceased Mrs. Chatterton. Mrs. Chatterton was the antagonist of Five Little Peppers Midway (until the oh-so-unrealistic overnight reformation), but we never had any hint she had this great-niece.
Yet it was pretty obvious, in FLPGU, that everyone apparently had a fair bit of history with Charlotte, the way they were talking and acting. And because of Margaret Sidney’s limitations as a writer, I never understood just why everyone had such a big issue with Charlotte. We were just supposed to accept that they didn’t like her, and that Charlotte was a problem. How did they get to know her, when was this, what exactly did Charlotte do to them, why were they so offended by her existence?
Charlotte wasn’t Ms. Sidney’s only surprise, full-blown character. She was pretty fond of throwing other full-blown characters into her books, like the readers were expected to just immediately transition to these people and their history with the established characters. Just take all the friends the Peppers suddenly have in Midway, which opens five years after the first book. Having a gap between books in a series is still no excuse for tossing in fully-developed characters with a lot of unwritten, relevant backstory. You could at least explain in a narrative passage who this person is, without slowing anything down.
I did this myself. In the seventh novella-sized book of my now-shelved, handwritten We the Children of Atlantic City series, set in 1949, Kit suddenly has three exes who were never even mentioned in all that previous time. Not only that, but two of them, the older Robert and the same-aged Frankie, have a fair bit of history with everyone. And she likes the third, Peter, so much she drops everything and returns to England with him. Giant plot holes and discontinuity!
If you decide to create a new character like that, you can’t just drop him or her in out of thin air. Whether it be a series or single book, you must go back from the start, or a reasonable point before the surprise introduction, and write that character in. It requires the same amount of effort, possibly more, as it does when you decide to take a character out. It’s not just a matter of removing or adding characters. This more often than not impacts storylines too.
When I became an Armenophile in 1995, I decided to add the Kevorkian siblings to my handwritten magnum opus Cinnimin. Only they were dropped in fully-blown in September 1942, with no explanation of who they were or how they got there, only that Cinni apparently liked some girl named Rebecca enough to have lunch with her, and that there were two brothers named Levon and Shavash.
I’m actually glad I created the Kevorkians, particularly Levon, since prior to that, there wasn’t a really consistent, developed plot. Just a series of funny adventures. Giving Cinni a foreign love interest really ramped up the action. This was also before I’d created the older storyline of how her first boyfriend was the older, Jewish Barry, whom she dated on and off in secret for about a year and a half. It’s odd to read really old drafts and the shelved series, as though Levon didn’t exist, Cinni had never dated Barry, and Cinni had actually slept with Max.
It insults your readers when you drop in full-blown characters, instead of gradually introducing someone entirely new, or taking a little time to explain the history your characters have with a new addition. If we’re talking about a major character’s ex-boyfriends, whom she would’ve dated when the series was well underway, that’s a serious omission. Even more serious than dumping in a bunch of fully-developed high-society friends who aren’t really that important to the overall storyline.