Note; This is an opinion, do not take this as anything offensive or insulting. This is to help you avoid having an overpowered or overangsty character.
It is very fun to make your own character, or fan character, yes? ^^ I agree. But, sometimes, people don't go through character development the correct way, having their character or fan character result in being overpowered and/or undefeatable.
Sometimes, they do follow the right steps, but instead create a much more angsty, annoying, depressed character.
I will start off with the much more common character; Mary-Sues and Gary-Stus.
They are characters who have many abilities, skills, or powers. They may be loved by everybody, they may be undefeatable, they may be immortal and/or unstoppable, or anything else overpowering. If these characters might be in trouble, their true love will rescue them. Ay yi yi.
These characters are the types of characters that ruin the fun in character developing. Gary-Stus are like Mary-Sues, but in which 'girls crawl all over them for their hotness'.
To avoid making one, every character has a flaw and/or weakness. Nobody's perfect. If you want your character to stand out, be more imaginative and original.
Marty-Sams are characters who whine about how horrible they are. They are angsty and are quite annoying. If they love someone, they would whine about how amazing they are and how they themselves aren't.
To avoid making one, it is very easy~ Don't make a character that is as angsty and/or annoying as a Marty-Sam.
Tragic Backstories are overrated and have been done the most. Most of the time, they involve;
"Doctor Eggman destroying their village, killing their family, etc. and them getting revenge on him."
Not every character needs a sad backstory. Make it original and make it something that others don't do.
Recolors are the most common problems. It is when somebody takes an art template of Sonic, Shadow (Mostly Shadow), etc. and color over it, claiming it as their own. Recoloring is bad, but if you ask permission from the original artist of the picture and/or give them credit for making the original image, then that's good. If you don't, you're going to have to, or delete the image, since art theft is horrible and the art is copyrighted.
Bases are excellent to use if you can't draw from scratch. But, always remember to give credit.
Powers, Abilities and Skills for your character is optional. :D They vary in many awesome things, from morphing into animals to causing acid rain! What makes giving these powers, abilities, and skills to your character fun, is when you are original with it. Don't go with the common propaganda that Shadow uses like 'Chaos Beam', 'Chaos Blast', 'Chaos Control'...Be more imaginative and creative with these things. Elements are awesome, too, but don't use the basic four elements all the time.
That's it for this guide. >:3