Creating an original character is one of the more popular devices in fanfic.  When carefully done, such a character can provide a new and fresh perspective on an established universe.  But poorly crafted OC's can drive readers to drink.  Unfortunately, creating a GOOD original character is no easy task.  There are many things a writer needs to consider when creating an OC:

 1) Is it necessary?

Do you really need an OC in the story, or can you tell the story with just the canonical characters?  If the latter is the case, then don't add an OC, for he/she/it will be superfluous.  Remember, most people want to read about their favorite canonical characters, not about an OC.  It's for exactly these reasons that most "tenth member" stories fail:  they simply retell a story that has already been told with just the canonical characters, and they try to make their OC the main event.

 2) What is the character's origin?

In LOTR fanfiction, OC's can either be from Middle-Earth or from the 21st century.  There are advantages and disadvantages to both scenarios.

A character native to Middle-Earth is more likely to fit in smoothly with the canonical characters.  A reader can safely assume that such a character will have the knowledge and skills  that many members of the Fellowship would have.  That is, the reader can readily believe that this character could know horsemanship, herbcraft, various languages and/or various combat techniques.  A Middle-Earth native will enjoy the acceptance or at least tolerance of other characters.  The main drawback is that it takes a lot of work to write this type of character successfully, as it requires extensive and detailed knowledge of whatever race or culture the character belongs to.  For a sterling example of how to handle this kind of character, read "Captain, My Captain" by Isabeau of Greenlea.

By contrast, the main advantage to using a character from the 21st century is that the writer doesn't have to do as much research.  But there are many disadvantages.  The biggest is the need to somehow handle the language barrier.  NOBODY from Middle-Earth knows or speaks English, French, Dutch, or whichever language happens to be the OC's native tongue.  Thus, realistically speaking, the OC will need to find a way to somehow communicate with the locals-- without getting killed or locked up in somebody's dungeon.  For that's another little problem:  the folks of Middle-Earth tend to be a fairly mistrustful bunch, which is understandable since they are at war with a dangerous enemy, and since many of them, like the hobbits, come from insular communities.

Another problem is that the character will likely lack the skills needed to survive in Middle-Earth, much less successfully go adventuring there.  If he or she does have such skills, the writer needs to explain why.  How or where WOULD a 16-year-old from California learn to handle a sword, anyway?  Culture shock is going to be a HUGE problem for this character, as everything, from food to clothing to sexual taboos to religion is going to be VASTLY different from what they're used to.  Food can be especially problematic here:  what if the OC's system can't handle the local food?  How would she handle drinking ale?  Remember, in medieval societies, it was acceptable for even young children to drink alcohol.  And what is to stop an OC from getting sick?  Her immune system is tailored to handle the diseases of 21st century Earth, not those of Middle-Earth, which means she would be highly susceptible to the diseases there.

Thus, while creating a 21st century character is an extremely popular device, it is also one that is egregiously mishandled.  Too many writers gloss over the difficulties such a character would face, and blithely assume that she would have no or minimal trouble in adapting to life in Middle-Earth.  Check out "Time Will Tell" by Bombur Jo or "A Serpent in Paradise" by AEMI for some of the better fics involving 21st century characters.

 3)  What is the character's name?

 Unless you're writing a comedy, don't name an Elf "Jason".  That is NOT a convincing Elvish name.  For a serious work, use names that are either derived from the canon, or convincingly fit it.  A quick and dirty way to select names for Middle-Earth natives is to go to www.barrowdowns.com and use their Name Generator, which lists hobbitish, dwarvish, elvish , and orcish equivalents to modern names.

If you're determined to make up your own names, remember that most people from Middle-Earth have relatively short names, with two or three syllables.  Galadriel's name is an exception, as it has four syllables.  Also remember that only hobbits and some humans use last names.  Other beings might use sobriquets that seem to resemble last names, such as Thorin Oakenshield or Elrond Halfelven.  Such names apply to specific individuals, rather than families.

 Human characters are probably the most difficult to name, as their cultures show the greatest diversity of all the peoples of Middle-Earth.  Consider the number of human languages, for instance, compared to the number of elvish or dwarvish languages.  Thus, a human character should have a name in keeping with their ethnicity and culture.  In other words, don't name a character from Harad "Bob Goatlife".  A name like that is more suitable to a Breelander.  In most regions of Middle-Earth, the names are patterned after those used in medieval northern Europe.  Harad might be an exception to this rule, as it appears to be loosely based on India.

 4) How old is the character?

Give your character a reasonable age.  Far too many writers like to send 15-year-olds to accompany the Fellowship.  Don't.  It doesn't work.  "Lord of the Rings" is largely an "adults only" party.  Virtually all the main characters are adults, as are the most powerful ones.  The ONLY adolescent character of any importance is Pippin, who is 28 when the story starts.  As hobbits are not considered mature until they reach 33, he'd probably be comparable to a human of 16 or 17.  For Pippin, "Lord of the Rings" was a coming-of-age story, during which he learned responsibility and courage.

It is also worth noting that Elrond very nearly did not let Pippin join the Fellowship because of his age, as he thought Pippin was too young and irresponsible.  He relented ONLY because he perceived the close bond that Frodo and Pippin had, and decided that Pippin might be useful in terms of emotional or moral support.  A strange 14-year-old human girl could not make such a claim, and would most likely be kept at Rivendell, out of harm's way.  There is NO WAY Elrond would allow her to tag along with the Fellowship.

A very young character will have less experience and knowledge than an older one.  Take the example of a hypothetical human 16-year-old archer.  She has, AT MOST, ten years' experience with the bow, while Legolas has literally MILLENNIA.  Even if she has more natural ability than he (unlikely), there will still be aspects to archery that she hasn't learned yet, and that Legolas mastered centuries ago.  In Middle-Earth, innate talent takes you only so far.  Experience and training count for a lot.  Even here, in the real world, it takes years to truly learn a martial art, and even longer to get really good at it.  The same is true in Middle-Earth.

Another, related problem is the assumption that all the non-human characters mature at about the same rate as humans, thus resulting in fics involving 16-year-old elves who act suspiciously like teenage humans.  This assumption is incorrect.  These beings are very nearly immortal, and they age very slowly.  They also mature quite slowly.  As per the 10th book of the History of Middle-Earth, a 16-year-old Elf would be roughly comparable to a human between 7 and 10 years old.  I say "roughly", because while Elves' physical maturation is considerably slower than that of a human, their mental development is actually QUICKER.  For instance, they learn to speak during their first year.   By contrast, a  human typically learns to speak during their second or third year.    Dwarves and Ents also have longer lifespans and slower maturation rates than humans.  With such characters, it is probably safer to describe an OC as an "adolescent" or "older adult" than to try to come up with a specific age.

 5)  Is your character in love with somebody?

Think long and hard before writing any kind of romance, as it is probably the most overused and cliché-ridden genre.  It is VERY difficult to give a romance a fresh wrinkle.  No matter what the plot is, SOMEBODY is bound to have done it before in their fic.  This is especially true if the romance is between an OC and your favorite canon character.

One possible way to make this story work would be to stringently apply "real world" thinking to the plot.  Too many writers gloss over any and all possible obstacles and concentrate on the cuddling.  That isn't the way a relationship works, at least not over the long haul.

People in a relationship have common interests.  They might like the same books or movies, go to the same church or school, like to visit the same vacation spots, and so forth.  They have shared interests that they can talk about or do together.

So what would your OC do with Legolas BESIDES shoot orcs together?  What would they talk about?  Where would they go for a date?  Do they have friends in common-- besides the folks in the Fellowship, that is?  What personality traits do they have in common?  It's even more difficult to find common ground between Legolas and an OC if the latter hails from 21st century Earth.  Mutual curiosity about one another can only take a relationship so far.

Also consider the effects of culture clashes.  Remember, in the canon, there were two or three human/elf pairings, and there were no known cases of interbreeding that involved dwarves or hobbits.  As per the books, orcs and humans sometimes crossbred, but it's debatable if the humans involved had any real choice in the matter....  The barriers to intimate relationships between members of different races would be cultural or physiological or both.  Given how rare such pairings are in the canon, those barriers are also considerable-- and can NOT be brushed off in a story.

AN:  I'd like to thank Nath for her comments on Elvish development and Jay for her comments on some of the physical difficulties a 21st-century OC would face.