When one has a certain number of things in their story- two characters, six buildings, fifty-two chapters, three books, etc.- we have a tendency as humans to say "But wait! This other thing that people know about has this many objects too! Maybe I can link them together!"
There are two sides to a coin or Yin and Yang. There are six sides to your average die. There are fifty-two cards in your average playing card deck. There are threes throughout the bible and trinities just about everywhere you look. So being humans, creatures that naturally want to lump things in tidy groups, we want to be able to associate things with other things. The more marketable the property, the more association. As I love to do, let's look at Batman:
The Joker, association to the card, and clowns.
The riddler, association to puzzles and childish playthings
The Hatter, association to Alice in Wonderland and other childish fairytale books.
The Scarecrow, association to both the Scarecrow of the Wizard of Oz, and the main character of the Headless Horseman tale.
All of these associate with the market that best related to them at the time of their creation. these character properties all link to children, who were the comic's primary audience early on. (anyone who says these characters are still for children needs to remember that we're referring to, in order: a mass-murderer, an egocentric psychopath, a possibly paedophilic (depending on interpretation and writer) man specializing in brainwashing, and a man whose entire goal is usually terrifying people for the sake of studying how to scare people even more.) So we can see these themes. Further, there are sometimes gangs revolving around these characters' themes. The Hatter has had characters revolving around Alice in Wonderland as his gangs, the Joker has his goons (and a spinoff gang, the Jokerz, in Batman Beyond), the Riddler had Query, Echo, and some other goons depending on writer, and even Scarecrow has had a few friends, allies, cohorts, and gullible goons.
But let's go a step beyond the cover, shall we? Some have likened them to playing cards (The Joker and Harley Quinn being the Jokers of the deck, obviously, with Batman and his cohorts all playing individual cards), or perhaps liken all the characters to pieces of the Bat's psyche.
But we have to be careful!
When you keep relating a character to a specific theme, it gets a bit obvious. People start to predict. That might make sense if you're basing you've got a theme and the characters are based off of the seven deadly sins (see Fullmetal Alchemist), but when you've got a disjointed story and you only relate some things, you can frustrate an audience. Worse than going halfway, though, is becoming wearily predictable. If your character selections in the future can be narrowed by Occam's razor, you'll have to look hard to come up with new and surprising characters.
Just a bit of a warning for those who -like me- love to theme their characters.
Luckily for me, in the work I'm doing now, I'm making all (or at least most) of the characters relate to entirely different things. Or trying to. We'll see how it turns out! Perhaps I'll prove myself a hypocrite after all.