This was partly inspired by this article by =dantesgirl; I recommend you read that on too! Where her article concentrated on original universes, we're going to take a look at fan characters and how they fit (or don't fit) into their chosen universes. A fun resource for this is the Mary Sue Litmus Test, but keep in mind that it's not perfect; for example, Tsukino Usagi of Sailor Moon scores a whopping eighty-seven points, but she's still a very flawed, yet very likable character that spawned one of the most recognized and well-loved anime/manga franchises on the planet! Don't be afraid, then, of high scores - instead, concentrate on why your character scored high and go from there.

What Makes A Fandom Sue?

This is a debated and largely subjective thing; some think it's beauty or talent (not necessarily), some feel it's a past connection to a canon character (again, no), and some feel it's romance with a canon character (nope). Now, all of these factors can certainly contribute to Suedom, but they're not defining factors. It's more closely related to how you use these factors.

Power and Talent

A powerful or talented character needs to (A) have something unique to offer, and (B) not upset the balance of a fandom universe. A character faster than Sonic, a swordsman more skilled than Megaman X's Zero or Princess Bride's Westley and Inigo, a more skilled rookie driver than Speed Racer, or an Avatar character that can bend all four elements is a no-no. (For the last case, exceptions may be made if you're chronicling the life of a pre- or post-Aang Avatar.) What to do, then? Easy! Find a skill that's not being used, or, if that's not feasible, work the skill in in a way that fits into the universe. If a canon character is known to be the only one with a particular skill, try to avoid it. (Even this has some exceptions; if, for instance, Aang finds someone with traces of Air Nomad blood and Spirit Bends them to keep the art of Air Bending alive, fine. That may be one of the reasons that talent was introduced. But your character will not also be a Spirit Bender, cannot become an Air Bender if they're already another Bending type, and they will not be immediately proficient or as talented as Aang. In fact, they'd need weeks, months, even years of intensive training to be able to do much of anything, particularly since they weren't born with it.) Moderation is the key here; take a step back and determine if your character is overpowered or provides too much overlap. Beta readers can help a lot here, so find someone you trust with the task!

One thing - for every great power, try to establish a debilitating weakness. Sonic can't swim. X and Aang suffer from crises of confidence that let the bad guy get the upper hand. Even Superman has Kryptonite. These are things that balance a character and make them more interesting, so don't toss them aside.

What about members of elite groups?

It's not as bad as you might think. There are probably dozens upon dozens of Maverick Hunters. There are several surviving Gargoyle clans. It's made clear that there are Sailor Senshi for every planet out there and some of the stars. Just don't usurp a canon character's role.

Beauty

Hey, beautiful people exists, and in anime and comics they're more common than not. Go ahead if you want a pretty character; you're not hurting anything. It becomes a problem, of course, when the character stops traffic, causes accidents, is the first thing anyone notices in a crowd, and has hordes of the opposite gender (and some of the same) hanging all over them. (If nothing else, everyone has different standards of beauty - someone who likes full-figured girls with short hair won't go for your tall, willowy goddess with flowing hair.) Highly unusual traits may be used, but try to have a reason for them, and try to keep them under control. (Remember, a magical creature in a human world will want to blend in. If you want an elf in Manhattan, great! Go for it! But keep in mind she wouldn't reveal elfin characteristics to just anyone.) Recolors, of course, are out altogether. You'll get much more satisfaction and respect for designing and drawing your own character than you will for just slathering red paint all over Katara's hair and clothes.

What about character designs based on me?

Contrary to popular belief, there's not really anything inherently wrong with that; fancharacters are, after all, created with a healthy dose of wish fulfillment. Even in personality, every character you create will contain a small grain of you. It's not a dangerous, Sueish self-insert unless the character echoes your personality in every way and becomes too deeply important, supplanting main characters, becoming a focal point in the lives of all the characters you love, and besting all the characters you hate - and those last three points are deadly Sueisms even if the character looks and acts nothing like you. If you're short with glasses and brown hair and like popsicles, and make a slightly prettier short character with brown hair and glasses who likes popsicles, it's okay. Just don't let it get out of hand - those beta readers I mentioned will let you know.

Past Connection To A Canon Character

This, too, is fine. It's not as if the characters sprang into being at the beginning of the book, movie, game, or show! They have pasts, and presumably people were in those pasts. Elementary school friends, playground pals, even old flames - knock yourself out. The exceptions come, of course, in characters like Naruto or Harry Potter, whose pasts are made clear from the getgo; in other words, don't break with established canon. The murky depths of Usagi's grade school days are fine; friends from Toph's pre-Aang days are an iffier thing considering no one seemed to know the Bei Fong family even had a daughter. If you must attempt a friend for a character with a pre-structured past, do so with care.

Family members are of course another story. Usually, a character comes with a predetermined number of siblings and a long-lost one is unlikely to say the least. A cousin might work, but tread with caution; it might be best rewritten as a very close childhood friend.

Romance

This is probably the one thing that makes people scream the loudest - fanchar/canonchar romance. It's okay. As before, the key is to do it carefully. Real, lasting relationships do not commence with people falling into each other's arms at first sight; it needs to build and grow, and if you allow for this, your relationship should be okay. Take it slow. Let them become friends, learn to appreciate each other, and then - after circling each other for a while - then you can take the plunge. Develop it carefully and realistically, and FC/CC romance can be intriguing and fascinating if you do it well.

A word of caution - try not to break up established couples. Alas, WALL-E is probably not going to ditch EVE for your new character, no matter how sleek and glossy she is. (Showing a relationship slowly break down and the characters finding love elsewhere can be well done, but if a particular couple acts as any kind of focus for the canon story, you're best off leaving well enough alone.)

Conclusion?

So what makes a Sue? It's very, very hard to say; like so many other things, you know it when you see it. Take heart! Moderate your characters and listen to criticisms about them. And remember - Sueing is less about what your character is than how they affect the world you place them in. Keep it in mind, and have fun!

Some Fancharacters For Your Perusal

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