Fancharacters. We're all familiar with them. Most of us greatly dislike the majority of them. This is because a large number of them are written by people creating their first characters, who really have no clue how to create an original character in the first place, and don't realize that inserting a new character into an existing work actually requires more attention to balance to be successful, never mind the fact that the authors often have fictional crushes on at least one character in the work they are writing about. For this reason, some of the most frighteningly bad Mary-Sues are often found in fanfiction.
This can be avoided. Some fancharacters can be made extremely well. It isn't fair to count out all fancharacters, especially if they're written into a series that has a new guest every week, or has even been known to pick up new permanent cast when the writers feel like it. If you hope to write a good fancharacter, there are some simple things to bear in mind.