Name: With names, you don't want it to stand out too much and research is important. Alot of beginner writers are prone to go straight to the Japanese names, but it is possible for someone from over seas to join the Cannon group. Using your own name is fine too, but I find it best to look at Baby Name sights and find lists of popular surnames in different time periods. Go through the whole list, make a sublist of the ones you like best, then match the first names with the surnames and see which one sounds the best!
Age: This ones easy. But people still slip up! Making your character too young yet giving them certain skills, takes the attention away from the real prodigies of the show (and remember the show is based around them, not you) If part of your characters personality is that their intelligent, make it one that doesn't require much awe, but still keeps them useful as a cast member. Maybe a skill that the main character, or the love interest doesn't have. I.E - Winry Rockbell is a Mechanic, who is a perfect companion (friend or lover) for Edward Elric, who is a genius and an Alchemic prodigy, but since he has Prosthetic limbs needs the aid of someone else to repair him when he's damaged. Making her useful but not pulling the attention away from Ed.
I haven't found too many OC's who create older Oc's such as 30+ but if your going to design an OC of an older age, be sure they look the part. Being 25 yet carrying the youthful glow of a 15 year old is kind of strange. Especially if your thinking of setting that character with someone who looks alot older~.
Occupation: Okay. I'm sticking with FMA here. This one catches most people out. I'm not saying that being a young State Alchemist is wrong, but alot of people go for it. Being a State Alchemist at a good age of 20+ is the most common in the world of FMA, it'd be nice to see a character who has a normal job or if you wish Alchemy to be included, have them be under training to be a State Alchemist. But remember, I aint saying being a young State Alchemist is wrong, so please don't feel insulted. Feel free to put State Alchemist here if you so wish, it's your character. But if you do, make you your State Alchemy Title is well thought out and not the first few names that pop into your head. Think about your Alchemy clearly, what is it's scientific explanation of function, then try to describe that with a name~ (Edward doesn't really apply here though). I.E - Water Alchemy - Water Alchemist, Aqua Marine Alchemist, Base Flow Alchemist, Riparian Alchemist etc
Town/Country: Well a good number of FMA fans know about this. Research comes into play once again, make sure you know the place's your characters are coming from and make sure that it's possible for them to travel to Central if they don't live their. I would like to see some more characters from Xing, but it may be difficult for them because of the desert, I rarely see Ishballan OC's either, being from Aerugo would be interesting but remember that they have border restrictions and Creta isn't that much talked about so you could create your own political explanation on what's going on. Either that or have them be an illegal alien without papers.
Relatives: I speak for myself when I say alot of OC's have a dead parent(s). It's kinda natural, we give our characters tragic pasts so they have subconscious bond with the character of choice you want them to befriend, but if you do have angsty pasts, be careful with how you portray it. Having your character have the exact same experience as Ed and Al is too unoriginal, or trying to bring a loved one back to life and now have Automail. It's fine if you have the above, but just try not to have such a tragic past. Maybe your character fought in the Ishbal war so they can relate with most of the current Soldiers in Ishbal, or one of their parents died but of natural causes rather than being murdered or slane by Scar or something.
Story: A quick backstory. Make it snappy. In presentations, you yourself and the reader need to know exactly who this character is within a small amount of sentences, drag on too long and the reader will become bored and maybe you yourself will have trouble digesting it yourself. Keep it short and sweet so that if someone hands you off hand, you know it all well and wouldn't need any pauses to think.
Involvement: Why have you created this character? Are they just a love interest? They need to have a reason to exist in the stories universe, do they have severe hatred for someone/ a group of people and the protagonist changes their opinion? Does this character have a unique skill that the cannon character in question can find useful, that the cannon character has something that the character wants. Why do they exist? Or simply your OC just needs a friend?
Personality: Make them human! Don't just make them feel one emotion all the time, they're not Tinkerbell! One of the best characters in anime/manga I find are those who act as a human being would. Ask yourself how you would act in that situation (honestly!) and would your character do the same. One thing, try not to have them freak out about people thinking their short, it's Edwards' best known trait.
Strengths: It's fine having a strength that other Cannon characters have, but don't try to overshadow the character. Find different ways around it or find different strengths that maybe none of the main characters have. Like a nurse/training doctor in FMA. Don't see many of those around - I don't even think there is one! lol Even the power of an optimistic outlook is a great strength since most protagonists have alot to deal with and can sometimes have a negative view on the world. Be the strength that that character doesn't have and the two of them will work well together, or have a simular trait and the two can be rivals. It's how you go about it!
Weaknesses: Here's where I normally find alot of blanks or things that don't make sense. Listing a bunch of weaknesses doesn't make your OC better, then again not having any weakness is bad too. Keep it simple like, afraid of the dark, can't swim, afraid of heights. Something that gives them a human weakness but hopefully doesn't set them back. There's no point in having a story set in Atlantis with a character who can't swim, or a vampire afraid of the dark. Research the story, document the popular surroundings and try to work it out~ I also think being clutsy is a good weakness trait, I don't see alot of them.
Other: This is just for any important information that doesn't come under any subheadings. I.E - at some point Sarah will discover this which will make her realise that her and Jake aren't all that different. Or Katherine reminds Dean of his daughter who past which gives reason to why he's so protective of her. Key facts that add to your character that are important, but not 'she hates carrots because she doesn't like the colour orange'.
This is my favourite part! You go on websites and see quotes from all the cast, these things help greatly when it comes to character develop! Things that they repeatedly say that gives them dimension! Like 'True dat yo' - speaking street even though they are the complete opposite, gives the impression that the character is continuously trying to be cool and fit in but in a futile manor. Either that or memorable lines that you need to remember that your character says that makes a big difference in the story and to the character being told. 'Revenge and Justice, they're the same thing, it's just dogmatic nonsense that makes the killer feel like he's doing something good when really he's just taking peoples lives out of stone cold hatred.'
RESEARCH IS IMPORTANT! I can't stress this enough! If you're going to be sharing your OC/ fan faction with the public, you need to get your facts straight or their going to notice your mistakes~! Make sure have done deep research in the universe your writing in, make sure that key plot points to not make a scene in your story unrealistic and inconcievable, make sure you aren't breaking any rules!