There are two types of Sue. A Sue character and a Sue Handler. BUT a sue is not sexist, no, you can have a MARY sue, or a GARY sue. Either way, they both are frowned upon.
A Sue Character is someone who is always gonna be at the centre of attention. They will be super cool, pretty, and if people don’t fall at their feet IC, then they will be seen as someone not in the cool crowd. A Sue is generally a stereotype. They will be at the centre of every storyline or thread, and even if they aren't, they will make themselves known, sometimes even taking over a thread. A Sue has no faults, can kick anyone’s ass, and will look good doing it.
A Sue Handler is quite the same, only OOC, they will make any conversation about them, and kick up a fuss when things don’t go their way.