There are lots of other people writing about writing, so I try not to add to the pile unless I decide something needs to be said. In this case, though, I also wanted an excuse to talk about Old Man Henderson.
For starters, go read the story of Old Man Henderson right now. It’ll take some time, but it’s worth it, and today is the perfect day for it. Basically, it’s the story of a Call of Cthulu-type game in which a player responds to an arbitrary GM dead set on grimdark by creating the perfect character to foil…well, the entire setting. It has everything: projectile yachts, weaponized high-end surround sound systems, and Necronomiblunts.
Besides being generally awesome, though, Old Man Henderson has an important lesson for writers: characters should always bring chaos.* By this, I mean that every character introduced (unless they’re so minor they would count as an extra in Hollywood) should fundamentally upset and alter your readers’ expectations about where the story is going. Otherwise, they’re just filler, like Town Guard #3 or Serving Wench #2.
Admittedly, sometimes you need filler. If you try to give literally every character with a speaking role some sort of deep background and complex motivation, your story will never be over. You’ll be writing life. And while I love life, it makes for terrible fiction. There’s no plot, too many characters, and things happen for no dramatic reason at all. That said, though, any character with more than minor importance to the story should change it noticeably. If a reader can predict where a story is going before a major character is introduced, why bother introducing the character at all? Let the poor guy/girl sleep in.
*Not necessarily The Crawling Chaos itself, of course. That might overdo it, especially in a romance novel. Although I guess that kind of works in Haiyoru! Nyaruko San…