"You already know Mary Sue. Mary Sue is the perky, bright, helpful sixteen-year-old ensign who beams about the ship. Everyone on the ship likes Mary Sue, because Mary Sue is good at everything. Mary Sue is an engineer, a doctor in training, a good leader, an excellent cook, and is usually a beautiful singer. Mary Sue often has mental powers that may manifest themselves as telepathy, precognition, or magic. Her past is tragic, more so than any other character on the series. (Many Mary Sues have a backstory that reads like a V.C. Andrews novel. This is a clue.)"
A bit of humor, a bit of background, in case your teen or pre-teen is reading on-line stories. Now you know the gist of 95% of them.