Amongst fanfiction writers, the Fix Fic is incredibly popular due to everybody having an opinion on how a particular canon should go. Don't like how they took your favorite character and Dropped a Bridge on Him? Nah, He's Just Hiding. Your OTP got ruined? Derail a few characters to get things on the right track. Wish the villain wasn't such a cad? Maybe he found that there was good in his heart after all. But how are you going to make this happen? This is when you call your local fanfiction repair business and request they send out a Fixer Sue.
The Fixer Sue is a unique entity in the Mary Sue class in that he or she or they was never meant as a character so much as a plot device. They may or may not represent the author, but what they do is use their magical plot-altering powers to get things "back on track" in the eyes of the author. The plot still bends over backward to accommodate the character, but instead of it being done for the greater glory of the author, it's to get to a particular end within the canon. Once the character is done fixing all the "problems", they will usually get unceremoniously removed (either by death, being Put on a Bus, or just getting downgraded to a minor character).
Fixer Sue can be just anything and everything so long as they get the results desired. A God-Mode Sue comes in and ceremoniously kills off the Love Dodecahedron outside of the OTP? Hey, that works. A Sympathetic Sue wangsts about how her (non)canon sister is depressed and unloved, inspiring the protagonist to choose her instead? That also works. This is also often done in a tongue-in-cheek manner (for example, a Parody Sue shows up and makes the hero realize how much he really hates extremely girly women and goes for the tomboyish childhood friend).
This can still happen within a canon itself, but it's quite rare. Still, maybe the author just ended up hating a particular path that was taken and decides to go head over heels during the Author's Saving Throw. Alternatively, the character might just exist as a walking Deus ex Machina and the story surrounding him or her is based around "fixing" every perceived social wrong. Characters prone to this include the Blithe Spirit, the Magical Negro, and The Drifter Walking the Earth.
Somewhat more believably, this may be combined with Peggy Sue (not a Mary Sue type, by the way), in which a canon character does this as a part of Setting Right What Once Went Wrong, or even as an unexpected coincidence or consequence of their actions.