First off, this is an OPINIONATED GUIDE. Don't take this as a troll threat or anything stupid. I'm just informing you all on how to make original and actually interesting fan-characters. If you don't like what I'm trying to do here. Stop reading now, because I don't like your face.
Let's start off with a few simple rules.
1. If your fan-character has any relation with the canon characters AT ALL, that MUST be eliminated. Don't have your character become Shadow's 9001st girlfriend. It's been done before, and better, so cut that shit off. You can have your fan-character be friends and or enemies with the canon characters, but that is the line. Be original and creative with your works.
2. If your character is a pure hedgehog, You need to mix it up a bit. Say that your fan-character is a hybrid of hedgehog and fox. ANYTHING but hedgehog. It's been done to death, it's a matter of originality, not that hedgehogs are bad.
3. If your character has any sort of special power, MAKE SURE IT IS NOT SOMETHING THAT MILLIONS OF OTHERS ALREADY HAVE! Give your characters something besides super speed and Chaos Control. It can involve the Chaos or Sol MacGuffins, but please have it be something other than generic emerald powers. Example: Your character, with each colour Emerald respectively, can alter her/his body in a certain aspect. That would be original enough.
Now, the basis of making a good fan-character is a good origin story. You know, were they came from, what they've experienced, etc. To reiterate, if it involves the canon characters in any vague way, REMOVE THAT, IMMEDIATELY. I don't care if you want to be Sonic's sister. Everyone does these days. Nobody wants to hear about your awesome adventures with your big brother, no one gives two damns. Make sure the backstory is also not a sob-story, where your character has been through shit, and has become a hardened warrior because of it. Cliché things like parents dying, raised by big bad, and any extremely tragic story like that is avoided. It's not fun to hear about how somebody's fan-character murdered a dragon when they were seven and was emotionally scarred by that. It's just plain generic.
Now, something I notice fan-characters lack is a sense of emotion and personality. The whole reason your fan-character is interesting is because of their emotions. Emotions should be set into a character from their past and opinion, if they have been handed everything to them and catered from head-to-toe, don't give them a battle-rugged attitude. It doesn't suit them at all, give them a personality that suits them, and if you have multiple characters, don't give them the same feelings, impressions and emotions. That's just lazy.
Special powers is what makes your character different from the norm. It helps to have them, but sometimes, there're generic powers, like fast speed. Having a unique ability allows your character to be stronger, in a way that doesn't make them OVER NINE THOUSAND EIGht hundred twenty-one... Now, elemental abilities are kinda good, but certain ones are pretty old and used up, such as fire, water, earth and ice, but some, such as metal, are pretty interesting. Giving your character specific abilities is even better. Take some time to think of some creative and interesting attacks for your character. You could also do some doodling, which is a very good idea for complicated attack patterns. Remember that weapons can also be used as a method of attack, such as blades or guns. but these can also be a bit boring, so vamp it up with energy such as plasma and the character's own abilities.
Which brings us to the matters of over-powerlessness. No character can be perfect, they all have to have some sort of weakness that makes sense due to events from their past or their main abilities. Giving your character a disease of some sort can also help. But there are such things as a generic weakness, oddly enough. There is the grade-A kryptonite, something VERY specific your character loses his powers against is a bad idea in theory and in work. You could rather try something as simple as a chronic case of a bad cold, or as daedalean as a specific illness. Giving your character a weakness is just as important as giving him powers, so don't ignore the fact that there is no such thing as perfection.
Something that is also influential is the appearance of your character. You should try drawing your character yourself in the best way you can, and if that's not your cup of tea, try the Furry Doll Maker, it's a simple and easy way to illustrate your character. You can also have someone else draw your character from either the Furry Doll Maker or a description. NEVER, EVER, IN THE HISTORY OF ALL THAT IS GREAT, RECOLOUR AN OFFICIAL PICTURE OF A CANON CHARACTER, OR EVEN WORSE, ANOTHER FAN-CHARACTER. It restricts your freedom to be creative, and it's pretty much theft. It just isn't a very good method of illustration. Don't do it, I don't care if you're a lazy git.
Last but not least, if you want people to know about your character quickly, a character sheet is the way to go! They look a bit like this:
Y'know, it's really simple stuff like that. Use that character sheet as a basis, and keep adding to it, keep it short and sweet, and most important of all, save it in a folder on your computer. You never know how easy it is to lose a character sheet in a forum post. Character sheets are necessary to keep basic information and tell people about your character in a organized paragraph. You should definitely use it during roleplays.
These are the simple basics to it all. I know it may seem like a lot, but once you finish it all, it's going to seem extremely quick, that's because it is. I encourage you to use my guide to your advantage and learn from it. With this help, you can have some pretty great creations. I know and I apologize for a bit of anger at the beginning, but that's just me venting. Try to ignore that and enjoy these tips and helpful hints.