You see, there's a term for the type of character mentioned above. It's called a "Mary Sue," or if the character is male, a "Marty Stu," "Gary Stu," "Harry Stu," or some other variation of the name. "Mary Sue" stories are so abundant that they make up their own category of fanfiction, and unless the author is intentionally writing such a story for humor's sake, they're usually unbearable to read. The reason for this is that a Mary Sue is not just a self-insertion (which is perfectly acceptable if done carefully), but an idealized self-insertion -- a version of the author that is extremely attractive, popular, multi-talented, always right, and completely lacking in flaws. In other words, not a real character. Did I also mention that she's typically the protagonist, and that you're supposed to follow her throughout the story and be totally invested in her? Yeah, try getting through fifty chapters of that.
Now I know what you're thinking: "So what if people write characters like that? It's just fanfiction." That's where my concerns started to pop up, because even though the term "Mary Sue" originated from fanfiction, it's not exclusive to that. I won't name names (a few are probably springing to mind anyway), but there are several highly detested Mary Sues in professionally written works of literature. All of the other characters adore them, they save the day every time, they don't develop or learn much over the course of the story, and they hardly have to work to get what they want. Not only does that make for a poorly-written character, but it can ruin your entire story.
So how do you determine if a major character in your book is a Mary Sue? As I discovered the other day, that's not so easy to do. The problem is that while we know what a Mary Sue generally is, there's no cookie cutter definition out there. Worse, there are many different types of Mary Sues that each have their own list of common tropes. It's okay for your character to follow some of those tropes, but you don't want him or her to follow too many. Some of the better Sue definitions and trend lists that I've found are on Urbandictionary.com, if you want to look through those. If not, here are a few traits to watch for:
- Having a very strange natural hair and/or eye color for no reason (for instance, a normal human having naturally orange eyes and pink hair just because the author likes those colors)
- Having a name that's uncommon for the character's culture, for no reason (a white Canadian who's named Yuriko just because the author likes that name)
- Having multiple nicknames for no reason
- Wearing clothes that are unusual for the setting or time period, for no reason (a modern-day African American who wears a kimono just because the author likes kimonos)
- Being described in far greater detail than any other character
- Being instinctively loved by all animals and/or children
- Succeeding at virtually everything they try, even though they've never tried it before
- Being able to learn new skills unusually quickly
- Being immune (for no reason) to weaknesses common to the character's species (a human who's immortal or can breathe under water "just because")
- Being able to dispatch entire armies of enemies single-handed
- Always saving other characters who should be able to defend themselves
- Being able to win every argument and convince every opponent to change their viewpoint
- Being romantically pursued by virtually every other character
- Being romantically interested in virtually every other character
- Being envied by virtually every other character
- Having a profession or being in an organization that they are way to young to be involved in ("She was accepted into the Air Force at age 13 because she's THAT good at flying planes!")
- Being immediately forgiven for every mistake they make, no matter how serious
- Routinely being let off easy and/or rewarded for breaking the rules
The key to all of those traits, as I gathered while compiling the list, is that they are so just because the author wants them to be so. There is no sufficient explanation given for them in the story, and they don't have any bearing on the plot or the character's development. While my characters that I was concerned about do exhibit a couple of the above traits, I feel that their backstories, their roles in their respective stories, and the events in their subplots justify them.
For instance, a character in my sci-fi/fantasy series is unusually fast at developing mental skills, such as learning new languages and solving puzzles, because their entire storyline revolves around them having that ability. Inversely, they're not very good at developing physical skills, whereas a Mary Sue would excel in both areas and the story would never address why or how.
So what makes a Mary Sue? This may sound obvious, but I think the answer is plainly and simply the author -- rather, the author's ability to write. If you put the thought and effort into creating a three-dimensional cast of characters with compelling backgrounds who develop naturally throughout your story, you should be alright. If you name every other character after yourself and have them beat up everyone you don't like with no repercussions, you might want to consider a revision.
And for the record, I've definitely never put any smut in any of my fanfics.