Depending on the character they can be good or bad.
Good Example:
http://img.photobucket.com/albums/v208/Senku/Drawings/Senya/ItsSenya.png
Bad Example:
http://img10.imageshack.us/img10/3760/smscharvd0.png
Now you may think, hey wait a minute, Senku's just posting one of his characters and calling it good because it belongs to him. Well, Senya may be my character but she CAN fit a good example, looks of course aren't what always make a character a character, it only enhances it, personality is what really brings out a character, and their aptitudes and such. Like a for Senya, she's a good swimmer, she's a bit on the overly happy and slightly hyperactive side, she will even try to look after her brother when he's in trouble. This makes a decent character, although looks could be slightly better. As for SMS, lets see, like Senya he resembles Sonic a lot, and his aptitudes are pretty much, "I'm the best at everything." That there makes a bad fan character. I wish I had another example of a good character other than my characters but I don't really know many good ones, plus mine aren't exactly the greatest either.