As you probably know already, characters are a VITAL part of fictional writing. Without at least 4 characters, the story would be flat.
4 characters... 4 VITAL characters.
These are the must-haves... However, to make your story more diverse and interesting, you will probably have several more of these.
Your main character (abbreviated: m/c or mc)... This one is pretty obvious. He/she will be the main person the story is about. E.g. Harry Potter in Harry Potter and the Sorcerers Stone.
Your antagonist... Some people take the role of the antagonist and roll it together with the villain. It works. It just does. The job of the antagonist is to... Well... Antagonize. He/she is supposed to make life miserable for your main character, and the characters that are helping/related to your main character. E.g. Severus Snape in Harry Potter and the Sorcerers Stone.
Your sidekick... The role of the sidekick in a story is often played by a best friend or a sibling. You may have more than one sidekick. And sometimes, the sidekick goes from being a sidekick to being one of the main characters. **collective gasp** And, I know... You're going to kill me, because we love these characters so much... (IT'S THE PERFECT EXAMPLE THOUGHHHH) E.g. Ron Weasley and Hermione Granger in Harry Potter and the Sorcerers Stone.
Last, but certainly not least: your villain... This is almost as self-explanatory as the main character, but I am still going to go through it... Your villain is almost as important, if not more important than your main character. It is also the hardest to develop. Most readers love a villain that they can sympathize with. Everyone loves a good backstory. When creating/developing your villain, you want to be EXTREMELY careful. With a diverse backstory, lots of other characters, and an absolutely fabulous plot-line, it can be very easy to let the smallest of details slide. And you definitely don't want that to happen with your villain. And, yet again, my example is from Harry Potter... Well, it has everything... E.g. Lord Voldemort (previously known as Tom Riddle to some) in Harry Potter and the Halfblood Prince (this one is different than the others, because we finally get his backstory in this book).
A few weeks ago, I was scrolling through one of my favorite blogs, and I found this handy list of questions to ask while creating your character (it's also great for keeping track of details **cough cough**. Great job, RWD!
Character Profile: 72 Questions