Like everyone else who started writing by composing fanfiction, I was conditioned by my early teens to recoil and cringe at the phrase “Mary Sue.” While many writers (especially very young ones) love writing about beautiful, brilliant, well-loved characters who get everything they want with minimal effort, very few readers enjoy reading about them.
In all kinds of fiction, readers want to learn about realistic, well-rounded characters who must face their own inner demons as well as whatever external conflicts await them. Portraying your characters’ personal flaws is an important part of developing them. Although some of your characters might overcome some of their flaws over the course of the story, many of these foibles will remain part of their personality throughout the narrative.
When deciding what kinds of flaws your characters will have, try to include at least a few flaws that will impede their ability to get what they want over the course of the story. This will make your characters’ development crucial to the progression of the plot, and your readers will find themselves even more engaged in both your story and your characters. For example, the plot of Jane Austen’s Pride and Prejudice hinges in part on Mr. Darcy’s ability to overcome his upper-class pretensions and help the Bennets out. The reader has a strong motivation to get invested in Mr. Darcy’s personal development because it is essential to his and Eliza’s getting what they want at the end of the story. It may be his good characteristics that attract Eliza, but it is his flaws which endear him to the reader and make him such a beloved character centuries later.
Flawless writing does not feature flawless characters. Develop your characters to be fallible creatures with well-rounded personalities, and make sure that their flaws function in some way as obstacles to the successful resolution of your story. This will keep your readers engaged with your story and help them get invested in your characters.