Original Character Treatise:
In fanfiction we place original characters that add our own original flavor to the overall story, and possible theme. However, whenever original characters (OCs) are introduced, a great amount of care must be taken into consideration. The reason why I say this is because far too many times I have seen 'original' characters that have come out of nowhere and have not only usurped an already established character, but the purpose as well.
The one great pearl of wisdom when it comes to original characters is to make sure that their involvement is not intrinsically linked to the main story.
The fact is, people read fanfiction stories to read about characters from that particular franchise. Not your 'original' characters. While they do have their place, their importance should never override the importance of established characters. Granted, there are outliers; like World of Warcraft, where many characters write their own stories that can exist in parallel to the overall written story (as poor as it is, but I digress).
Case in point; in Malaradark's story Dark Energy, there is a character by the name of Gellian Osco. (Used with author's permission) Mentally unbalanced and sick, but unbelievably intelligent. However, her involvement in the overall story is all behind the scenes. Here are a few facts about her; (Spoilers for ME1 apply)
She was taken under Benezia's wing and considers her a friend, if not the only friend she has.
She was there on Therum when Shepard and Co. saved Liara.
She was there on Noveria when Benezia was killed and tried to kill Shepard for it (but failed, obviously) and is still trying to do so, indirectly.
Her presence was instrumental on Virmire when Saren was trying to clone Krogan without the Genophage, however, it still ended up being destroyed.
See the pattern?
It's all in the background and doesn't change the established canon of a story. As Malaradark and I spoke, she noted that had Osco's presence been on Virmire and both Kaidan and Ashley could have been saved because of it, it would have turned Dark Energy into an Alternate Universe; which was not the author's intention.
OCs can add some interesting flavor and in universes such as Mass Effect, there's so much potential. However, take care. Remember, OCs are icing on the cake of the adventures of canon characters. Eating nothing but icing is not only bad for your tummy and teeth, there's not much substance.