Just some thoughts, especially for the new writers on the list, 
but I'm sure the more experienced members will be able to add their own 
wisdom...  I'll be using Ranma 1/2 as the example, since it's more or 
less our lingua franca, but much of this should be generally applicable.
	There are three main classes of new characters that appear in 
fanfics. 
	1.  Villain of the Week.  A new antagonist for our hero(es) to 
fight.  While as the name implies most of these are one-shot characters, 
in more campaign-oriented series such as Sailor Moon the new baddies can 
cover an entire series of stories.  An example off the top of my head:  
the Ace of Hearts, from the "Ace of Hearts" story.
	2.  Co-Protagonist.  A character who is effectively "co-starring" 
in the story with the Ranma characters.  Often these are stand-ins for 
the author (and friends) or a favorite character from the author's 
original work.  Example:  Twister, from "Twisted Path".
	3.  New supporting cast.  Just about everyone else.  Often 
created to fill a niche ("I wonder why we never see a police officer in 
Nerima?") or as adjuncts to the first two types.  Note that VOTW and CPs 
can be moved into this category if they stick around after their main 
storyline is completed.  Example:  Just about every story has at least 
one for a cameo.
	
	Power/Skill Levels:  Villains of the Week are almost invariably 
more powerful/skilled than Ranma...or at least appear to be during the 
first battle.  Usually they have some sort of edge which the hero must 
either eliminate or work to overcome.  VOTW that are *really* more 
powerful than the main character should either have a vital and 
believable weakness, or provide some excuse for the heroes to team up on 
him. 
	Co-Protagonists should generally be about the level of Ranma and 
Ryouga, or have some special talent/power that allows her to keep up.  
Note that the CP can be *considerably* less powerful/good at fighting if 
the story is skewed away from combat.  Something to keep in mind;  Ranma 
and Ryouga are as good as they are at fighting because they sacrificed 
normal childhoods to get that way.  A CP shouldn't be much better at the 
martial arts unless they also didn't have much of a life other than 
training. 
	Supporting cast of course can be at any power level necessary, 
though they are generally less powerful than the main characters, or have 
plot reasons for keeping themselves in the background.  (For all we know, 
Doc Toufuu could whip Ranma's butt six ways from Sunday, but he prefers 
the quiet life of a simple bonesetter, thank you very much.)

	Things To Do
	1. Make a character biography sheet.  This is especially 
important for Co-Protagonists, but Villains of the Week will need it 
too.  (Especially as they may not get enough time on screen to fully 
develop their personality.)  Include a physical description, a basic 
overview of her personality, important quirks and the events that spawned 
them, combat style if any, and other things you'd ask about someone the 
first time you met them.  Then a few paragraphs describing the 
character's life in chronological sequence up to her arrival in the story.
	2.  Determine baseline motives and reactions.  Why is this person 
in Nerima, and interacting with the Ranma characters?  Who would they 
react favorably to, unfavorably, just be boggled by?  Ask yourself what 
the most likely events are and how he'd deal with them.
	3.  Show the bio sheet to a friend.  Get them to ask any 
questions that come to mind, and write down the answers.  Note any gut 
reactions.  If your friend says "this guy sounds like a complete SOB" and 
you were attempting to make the character likable, you may want to modify 
him a bit, or change his role in the story.
	4.  Write the story.  now that you've put this much work into it, 
you might as well get use out of your character.

	Problems...
	1.  Excessive power levels.  This is not so much a problem with 
Villains of the Week as you can always give Ranma a quick one-time 
equalizer if all else fails.  Co-Protagonists are another story.  Many 
fans of Ranma prefer *him* to be the kick-butt hero, so having the CP 
being able to wipe the floor with him doesn't sit well.  Besides, if the 
CP is able to wipe the floor with the toughest regular Ranma cast, then 
new foes have to be introduced just for her, which makes the regular cast 
kind of redundant, which leads to...
	2.  "Too cool for this series".  This happens when the new 
characters have basically taken over the story and the Ranma cast is 
relegated to spectators or the butt of jokes.  If the word "Ranma" can 
be replaced by the word "John" without changing the sense of the story, 
you've gone too far.  
	3. Bunching.  Introducing too many new characters too fast can 
leave the reader confused and disinterested.  It's tempting to introduce 
the new Co-Protagonist, his arch-enemy, their love interests, trainers 
and odd relatives all in one slew.  But it's better to let them trickle 
in one or two at a time, with whole chapters between introductions to 
develop the characters you already have.
	4.  Same old, same old.  This is most noticable with Villains of 
the Week.  Takahashi-sensei and the anime writers did a terrific job of 
providing lots of VOTW for Ranma to fight; it takes some doing to come up 
with one that's different enough to hold interest.  Take some time to 
find an original twist or two; perhaps *this* VOTW is really after Kumon 
Ryuu, but is attacking Ranma because Kumon was using that name when the 
villain met him.
	And connecting new characters with less-overused regular cast 
members helps too.  ("Gee, Gos, I didn't know you had a cousin.  What 
gives with the spooky makeup?")


	Hope this has been of some help....

SKJAM!
http://www.geocities.com/Tokyo/Flats/5990