You have never met a perfect person. Even that fashionable girl with all the right accessories, or that Adonis who buys a latte every morning and somehow still has washboard abs, is flawed. All human beings are flawed. That's why the characters you create on the page have to be flawed, too.
Some flaws are pretty common, and relatively small. Your character might bite their nails, or forget to put their keys somewhere safe. Maybe they lose their phone a lot, I don't know. If you're writing a well-crafted character, you're writing one that has flaws. But many authors take this one step further. Many authors use an important plot device to move their stories forward.
Many writers create characters with a fatal flaw.
There's a school of belief that says we all have at least one terrible flaw, something that keeps us from being happy or achieving our goals or just moving forward in life. A fatal flaw is a mistake that we repeat again and again and again, some characteristic that leads us into mishaps, failed relationships and ugly situations. I have many fatal flaws.
Does your main character have one?
Characters who have a fatal flaw exhibit the same behavior repeatedly, continuously making the same mistake. The mistake may not necessary repeat itself in the exact same way. A character who talks too much and reveals too much, for instance, may shoot their mouth off in class one day. The next week, maybe they go on a Twitter rant. It's two sides of the same flaw. Many characters are written with fatal flaws as part of their make-up for one simple reason: to eventually beat it.
At some point, the character who has a fatal flaw begins to learn from their repeated mistakes. At some point, they start to see their own pattern and actively fight against it. The moment of greatness comes when the character is faced with a situation which would ordinarily bring out their flaw.
In the example used above, the talkative character might be asked to give a speech about something they feel passionately about. By giving a short, to-the-point speech -- or better yet turning down the honor entirely -- the character shows that yes, they are growing and developing. And yes, they have beaten their own fatal flaw. Inevitably, this will lead the character to some good conclusion. Because they kept their mouth shut, their main love interest suddenly looks at them with new eyes, for example. A whole new world of possibility opens up before them.
The fatal flaw is a very useful fiction technique, and some of the greatest literary characters have at least one fatal flaw. Scarlett O'Hara, Anne Shirley, Emma Woodhouse -- the list goes on and on. A lot of plot develops because of the flaw -- the character continues to find themselves in trouble due to their own flaw. When the character finally rises above this flaw it's truly exciting and compelling, when it's written well. Overcoming a fatal flaw can, in fact, be the entire plot of a novel, if the flaw is terrible enough and the character likable enough.
So give your next main character a fatal flaw, and teach them how to overcome it through the pages of your writing.