Throughout fanfiction, there is no character so reviled and yet so pervasive as the Mary Sue.

Mary Sue: *walks in combing her long, raven-colored locks and fluttering the pretty lashes of her deep violet eyes* Did someone call me?

Discussions abound regarding the character's origins, her psychological significance, her attributes and features, her role as a post-modern feminist restructuring of a previously boys-only heroic genre.

Mary Sue: *aside to MM* My what now?

MM: *aside to MS* Why you're here.

Mary Sue: Oh! I'm here to save the day! *swishes sword around expertly* I was trained in secret from an early age on weaponry and the arts of war. Also, I am a special snowflake.

MM: I see.

Mary Sues are a common character type in both professional fiction and fanfiction. They can evidence themselves as author-insertions ("She's like me! And she's hanging out with Frodo and Aragorn!"), wish-fulfillments ("Sharendriel the Fair is beloved of everyone who meets her! Except those mean villains!"), or sometimes as idealized mates who end up in relationships with author inserts ("Krista, an exotic beauty from Kleverston, was unsurpassed in her hand-to-hand combat skills and intelligence, but her heart belonged to Steve, the misunderstood gamer genius.").

MS: That Steve sounds like quite a catch!

MM: Of course he does, dear. You were designed to be his perfect girlfriend.

MS: Hooray!

MM: *facepalm*

There are many physical features common to Mary Sues, as well as certain commonalities to their backstories. Gather more than a few of these in one place, and it's often a sign that an author is spending too much time on one character, and not enough on the actual story.

Some examples: unusual eye color (often violet); long, flowing hair, which is described in detail (really, any character whose hair is mentioned more than twice unless the hair is physically picking up and eating another character, because that would be kind of cool); names that are a takeoff on the author's name, or his/her middle name; an angsty childhood, usually involving the loss of one or both parents and quite often abuse at the hands of the new caregivers; special powers that allow him/her to communicate with animals such as horses or birds; telepathy; a twin, clone, or close sibling of the same gender (especially if the author writes more tales about the twin); a heretofore unknown familial relationship with already-existent characters; a burgeoning romantic relationship with the existing character(s) of the author's preference, despite any already present relationships; special abilities that overshadow similar abilities among existing characters (smarter, faster, stronger, better engineer); a certain "charming" klutziness that results in minor or major disasters but is rectified by the end of the piece; a heroically sacrificial "death" which is often fixed at the end or in the sequel, but fixed or not, the character is mourned by all, even to the point of reforming a villain by this noble gesture. Also, Mary Sues have a habit of being mentioned or described in story titles.

MS: Wow! I'm a very busy girl!

MM: If it makes you feel any better, you tend to do this over multiple stories.

MS: I get multiple stories?

MM: Yes, because a big sign that an author has a Mary Sue is the author's apparent abiding love for the character. If there's a fanfic series all about one character, it's probably you.

MS: You love me! You really love me!

With all those traits in hand, a character can still be written as a reasonably down-to-earth and well-rounded character.

MS: I can?

MM: Yep.

MS: Why?

MM: Because well-rounded characters are good.

MS: *pause* Does that make me not good?

MM: ...

However, the difficulty in doing so tends to elude inexperienced writers, and even writers who have been at their craft for decades. Mary Sues are poor excuses for characters, but once a niche is located, they sell to people who want to pretend to be those same characters: literature as catharsis and/or escape for the reader.

MS: So I'm bad.

MM: You're not bad.

MS: But I'm not well-rounded.

MM: Not usually, no

MS: *bounces out and bounces back in with a short wig and brown contact lenses* I'm fixed now, see? Well-rounded.

MM: You're wearing a wig and contacts.

MS: But I don't have long flowing hair or violet eyes.

MM: *taps foot* Were you still sold into slavery by your foster parents while your twin sister was raised in secrecy far away?