Now consider the difference between Spanner's Leila Renata Shelley and ludicrous badfic creations like, I dunno, let's call her Ebony Dark'ness Dementia Raven Way (an extreme example for the sake of illustration). Leila: languid pale beauty with black hair elegantly bobbed, violet eyes, and ninja training (long story). Raven: well, "goff" (sic), with (and I quote) "long ebony black hair with purple streaks and red tips that reaches my mid-back and icy blue eyes like limpid tears"; her catch phrase is (and I quote), "Why couldn't Satan have made me less beautiful? It's a fucking curse!" To which Leila replies, "I don't care how beautiful Satan made you if you're a total bleedin' git." Sure, Leila has been known to cut herself and attempt suicide (long story), but she has no tolerance for "posettes" (feminine form of "poser"), the Rocker version of what the PPCs call "fangirls".
Now here is why I picked Ebony Dark'ness Dementia Raven Way as my counterexample: she is the supreme posette of fanfiction. Oh, and her hair is purple. Apparently even silky raven locks (to swipe a notorious bit of urple prose) such as Leila's isn't "goffik" enough for her.
Here's the scenario: After coming back from the dead after being killed by yet another Mary Sue hunter, Ebony returns to Hogwarts only to find that one Leila Renata Shelley has stolen not just her position in Slytherin House but her beloved Draco Malfoy as well. (Now here's a scary thought: if there's any character in any continuity who can encourage Draco Malfoy to Voldemort levels of evil, it's Leila Shelley.) Ebony's all "goffed" up and listening to Good Charlotte as she plots her revenge when Leila comes to her:
Leila: (switches off Ebony's iPod) "What in bloody hell is that supposed to be?"
Ebony: "That's goffik, bitch!"
Leila: "By the way, the word is 'gothic,' and Good Charlotte are not the least bit gothic. Real goths can't stand that mopey emo crap."
Ebony: "Let's hear you explain the difference."
Leila: "Gothic rock is melancholy and deep. Emo rock is wangsty and naff." (Note: You can actually hear the Author straining for words...)
Ebony: "So what are you listening to, huh?"
Leila: (leans against the wall, smiles romantically upward) "Right now I'm in love with Emilie Autumn."
Ebony: "Ew! That's not even rock!"
Leila: (crosses her arms, smiles contemptuously) "So how'd you earn your credentials as an expert on rock? Huh, 'Goffy'?"
Ebony: (shakes her urple hair indignantly) "Same to you, you bloody cunt!"
Leila: (shakes her head sadly) "Ever heard of the Damned?"
Ebony: (confused) "Who the fuck are they?"
Leila: (frowns) "Figures. They invented gothic rock. Did you know that Rat Scabies himself is my godfather?"
Ebony: "You're lyin'!"
Leila: "And you're posin'."
Ebony: (spots Draco coming) "Draco, my love! Help me! Get this bitch away—"
Draco: (takes Leila's arm in his) "Leila darling, is this stupid git twisting your knickers again?"
Ebony: "No — Draco—"
Leila: "Shut your gob, slob. Draco belongs to me now." (looks lasciviously at Draco) "Shall we go and do some... 'evil' now?"
Draco: "Your place or mine?" (waves contemptuously to Ebony) "Toodle-oo, Mary Sue." (leaves with Leila for Marty McFly's time-travelling DeLorean, making out the entire way)
...and they leave Ebony sputtering. What's a heartbroken Mary Sue to do? Of course you know what Ebony Dark'ness Dementia Raven Way will do: call her Twu Wuv, Gerard Way — over and over and over, until once again he's annoyed enough to call in the Mary Sue hunters after her.
*clears throat* Anyway, the moral of this story is, if you're a character whose author doesn't detach themself from you that you're sufficiently got in, you'll suck hard enough that a genuinely interesting character can swoop in from some other continuity and swipe the love interest you yourself swiped from canon. Yeah.