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No Mary-Sues/Gary-Stu
A Mary-Sue is basically a character who is too perfect (especially when it comes to appearance) and has little depth. A Gary-Stu is the male equivalent. Often, the profile of a Mary-Sue character focuses on physical appearance and achievements rather than personality and character motivation. It is a form of godmoding, though godmoding tends to focus more on skills, talents, and powers rather than appearance.
For this particular guild though, it is easy to fall into the "over powered" and "over linked" classification of a Mary or Gary Sue. Things such as having unnatural or insane powers...having abilities that allow the character to solve or do anything...being related or linked to cannon characters that throws off the cannon story line, etc.
Here's an example of a Mary-Sue profile:
Destiny Love is 5'6 and weighs 100 lbs. She has platinum blonde hair and sparkling blue eyes. She is a human but she is on the side of the mutants and is good friends with Xavier. She goes to Xavier's school and is captain of their cheerleading squad. She has a crush on Bobby Drake, but he doesn't know it. Right know, she is wearing a pink tank top from Abercrombie and Fitch and a white miniskirt with ruffles. She is wearing purple eyeshadow and red lipstick.
Notice how the person begins by mentioning appearance very specifically. The character is severely underweight, but somehow the person considers that attractive. Despite being a teenaged human, her character somehow managed to not only befriend Xavier but also gain admission to the school despite having no powers at all. She also has a crush on a canon character, and the roleplayer probably intends on having her character date Bobby and fulfill the roleplayer's own fantasy rather than be realistic. She keeps focusing on appearance and unlikely circumstances. The name of her character is very unlikely and too much of a cutesy name. It also has nothing to do with the character herself; it just "sounds pretty." Please avoid all these mistakes.
Names
Try to name your character something reasonable. It doesn't have to have some kind of meaning, though that would help. It doesn't have to be fancy. After all, Jean Grey and Scott Summers are examples of names of characters in X-men, and they're not fancy. The mutant nickname (like Cyclops or Wolverine) should have some meaning, but it's usually not hard to think of a name. There's a difference between their mutant name and their real name. Please don't just give your character a weird name just because it sounds cool. In reality, it usually sounds pretty stupid. I've heard of names like Butterfly Fire and Jade Diamond. Who the hell would name their kids that? You don't have to give your character a really common name, but try not to go overboard on the weirdness.
The Tortured Soul
Characters that are orphans, abused by their parents, murderers, runaways, and/or have some kind of overblown, overdramatic past. Although it is true that in real life, people have been abused by their parents, not everyone in the world has had some great trauma in their life. There are other ways to make your character interesting than to add melodramatic event to their past.
Checking your character:
Universal Mary Sue Limit Test
Pretty thorough Mary Sue Test
Same as above, printer friendly version
For more information, and citations:
Mary Sue Wikipedia
How to avoid making a Mary Sue
18 ways to spot a Mary Sue
A Mary-Sue is basically a character who is too perfect (especially when it comes to appearance) and has little depth. A Gary-Stu is the male equivalent. Often, the profile of a Mary-Sue character focuses on physical appearance and achievements rather than personality and character motivation. It is a form of godmoding, though godmoding tends to focus more on skills, talents, and powers rather than appearance.
For this particular guild though, it is easy to fall into the "over powered" and "over linked" classification of a Mary or Gary Sue. Things such as having unnatural or insane powers...having abilities that allow the character to solve or do anything...being related or linked to cannon characters that throws off the cannon story line, etc.
Here's an example of a Mary-Sue profile:
Destiny Love is 5'6 and weighs 100 lbs. She has platinum blonde hair and sparkling blue eyes. She is a human but she is on the side of the mutants and is good friends with Xavier. She goes to Xavier's school and is captain of their cheerleading squad. She has a crush on Bobby Drake, but he doesn't know it. Right know, she is wearing a pink tank top from Abercrombie and Fitch and a white miniskirt with ruffles. She is wearing purple eyeshadow and red lipstick.
Notice how the person begins by mentioning appearance very specifically. The character is severely underweight, but somehow the person considers that attractive. Despite being a teenaged human, her character somehow managed to not only befriend Xavier but also gain admission to the school despite having no powers at all. She also has a crush on a canon character, and the roleplayer probably intends on having her character date Bobby and fulfill the roleplayer's own fantasy rather than be realistic. She keeps focusing on appearance and unlikely circumstances. The name of her character is very unlikely and too much of a cutesy name. It also has nothing to do with the character herself; it just "sounds pretty." Please avoid all these mistakes.
Names
Try to name your character something reasonable. It doesn't have to have some kind of meaning, though that would help. It doesn't have to be fancy. After all, Jean Grey and Scott Summers are examples of names of characters in X-men, and they're not fancy. The mutant nickname (like Cyclops or Wolverine) should have some meaning, but it's usually not hard to think of a name. There's a difference between their mutant name and their real name. Please don't just give your character a weird name just because it sounds cool. In reality, it usually sounds pretty stupid. I've heard of names like Butterfly Fire and Jade Diamond. Who the hell would name their kids that? You don't have to give your character a really common name, but try not to go overboard on the weirdness.
The Tortured Soul
Characters that are orphans, abused by their parents, murderers, runaways, and/or have some kind of overblown, overdramatic past. Although it is true that in real life, people have been abused by their parents, not everyone in the world has had some great trauma in their life. There are other ways to make your character interesting than to add melodramatic event to their past.
Checking your character:
Universal Mary Sue Limit Test
Pretty thorough Mary Sue Test
Same as above, printer friendly version
For more information, and citations:
Mary Sue Wikipedia
How to avoid making a Mary Sue
18 ways to spot a Mary Sue