How to not write a Mary Sue

Canon Character Relationships

It's fine for your OC to have a significant connection to an existing character, but make sure it's a plausible one and that it doesn't interfere with that characters previously established personality, image and back story (ex: if a character is established as a misanthropic loner who's never experienced real companionship, don't make your OC his beloved best friend from back-in-the-day that he just happened to never mention before) and don't make your OC solely responsible for everything that fans like about the existing character (don't make her the girl who got Wolfgang Amadeus Mozart into music or the first person to ever suggest that Sherlock Holmes get into the detective business.)

• If you want your character to date a main character you like, okay, but be careful. Remember a lot of fans out there want the same thing for their characters too.

• Don't do Love at First Sight. It can happen in Real Life, but it's still thought of as discredited in fiction, and doing it for an original character will almost instantly get your character labeled as a Sue. Try to pace any relationship for realism.

• As a one-sided crush, Love at First Sight can work, but since most use it to immediately establish romantic relationships between two characters, this tends to get overlooked.

• Don't use Die for Our Ship to get rid of any existing love interests. That's another thing almost sure to mark your character as a Sue. If there is a confirmed love interest, just play it safe and leave her be.

• Similarly, don't break up a Canon couple or a popular Fanon couple or Fan-Preferred Couple just for the sake of getting your OC with your favorite canon character. Especially if there's no good reason for said couple to split, and no good reason for Canon Character to decide he/she wants to be with OC, this will almost certainly cause the audience to call foul.

• Don't incorporate romance into your story if it would overwhelm an otherwise non-romance-centric plot. Unless the story centers on romance, it has to be worked in so that it is organic to the plot. Otherwise it's going to stick out like a sore thumb. This is bad even when only canon characters are involved. Shoving an OC in there just makes it worse.

• Being related to a main character should be done carefully as well.

• If your character is a child of a main character, it had had better be done in a way that makes sense. Being the child of two same sex characters just isn't going to cut it unless it involves surrogates, adoption, being the child of one of them from their Last Het Romance, the setting allows that with some sort of Phlebotinum or you explain/find out that sort of magic exists in the canon.

• Being a long lost relative should also be plausible. If the main character is an orphan, you have plenty of opportunities. But even then don't just have the character pop up and announce the relation. What allowed this character to discover the main character? Why does it matter? Most people don't go around seeking out long-lost relatives, and even orphans don't always go looking for their surviving relatives.

• If your character is the long-lost orphan child of a relative of one of the canon character, consider how they know that they're related. How did they find out? Why do they believe it? Also consider how the rest of the family feels/felt about the character's parent; this may well affect how they react to that person's child showing up out of the blue.

• Also, you don't even have to bother with being a long lost relative if a main character is known to have a large family, or the series is legendary for having new relatives just drop in out of the blue. Just be one of the many cousins/cloned siblings/etc. that character is likely to have.

• Just be careful about having your OC being a Kid from the Future.

• Giving characters with no known parents or only one known parent doesn't automatically mean the parent that isn't seen is Killed Off for Real; think of other possible explanations for their absence before using death as the reason.

• If a canon character is noted as being the last of his or her race/species, especially if this is a major plot point, don't introduce a previously-unheard of family member who is the real last one standing.

• Don't have characters act differently around your character. Character Derailment is another discredited trope, so that also goes double for fanfiction. Characters should react to your character in a way that fits their personalities.

• If a canon character has stated that they don't want children, don't have them suddenly change their mind and get pregnant/knock someone up just so you can introduce an OC in the form of their child.

• If the character has gone to some form of high school/college equivalent, this might help justify the idea.

•If your original character is the child of an existing one, and the existing character is already at the top of their field (or worse, a grown-up child prodigy), your original character should never be better or more accomplished at what their parent does than they are. It's unlikely that your reader is going to find it cute that your made-up daughter of Sherlock Holmes is able to crack cases faster than him.