A writer who creates perfect, wangsty, or unrealistically talented characters who have no real personality or depth and often become the center of a fanfiction or original fiction, ruining whatever the plot was, if there was one to begin with.
Hint, hint: being allergic to Sundaes with spiders and pink, radiated vegetables in them from the Amazon at two o'clock in the afternoon is NOT a weakness.
Suethor: OMG!1!!11 ur so MEEN!!!1 i cnt blive u Sed THAt! OMGWTF!!!11 u nd ur rigting SUX BiCH!!1111one
Most popular Harry Potter Mary Sue of all time:
The American transfer student.
Suethor: LYKE OMGWTFBBQ!!!!111! U BETTR STOP TRASHIN' MY CHARRIES CUZ SHEZ A BADAZZ GURRL WHO CN KICK UR AZZ! WTF ARE YOU DOIN' ACTING LIKE A TOTAL BITCH TO ME AND MY CHARRIES? GO TO HELL, LYKE OMG!!! ELEVENTYONE!!
Reviewer: ...um, no....