... a full novel about the non-Sue Mayri Rosewind Syue? She might need renaming but the story sounds pretty good.

A sue is a character you want to be, that is the way they start out as a fantasy. An OC is character created in an attempt to be interesting to others. Thats how I'd put it.

I like your art, for starters. It's a good demonstrator of how Mary Sues and OCs can be alike, but different.

Mary Sues are, more or less, the ones the entire fic is about. If an author places so much emphasis on them, to the exclusion of canon and other characters, you have a Mary Sue on your hands. Spreading the action around, giving your OC faults and mistakes and a gradual change throughout the course of the story, helps prevent Mary Sue-ism from occurring.

It seems to me that it's not about what the character has, it's about how they got it. If an OC has had to work for hours to get merely proficient with a weapon, and they occasionally/frequently find someone better than them with it, they're realistic and well-portrayed. If a character simply picks up the weapon and is amazing at it, that's a Mary Sue.

I'm still slightly unsure on some of the finer points of spotting Sues, but my general definition right now is along the lines of "they're what it's all about." If someone doesn't like them, it's because that person isn't smart enough to see how amazing they are. If someone likes them romantically...well, of course they do! Weapons? They're a natural. Magic? Of course!