Used to negatively describe a female character, usually in a fanfiction (but sometimes in a Tv show or movie,) that can be just be very annoying because they are too perfect or there aren't any/many weaknesses in their personality. If they are a side character, they can sometimes take the spotlight. If they are a main character, it can make an uninteresting story. Sometimes turn into a joke. Also look at Marty-Stu.
This is a prime example of a Mary-Sue that I found on a writers page on fanfiction.
"Michelle Isabella Fenton
Danny Phantom's thirteen-year-old sister, who is two years younger than him. She is Danielle Phantom's identical twin and Nat Wolff's girlfriend. She is a Harry Potter style witch and a Aquamarine style mermaid. Michelle has dark brown hair and aquamarine blue eyes. She is an Avatar style water and airbender. Michelle is 25 percent Twilight vampire, which explains why people think she looks perfect. Her hair is elbow-length and she is 5'3. In her power form, she has a white bikini top, full white knee-length skirt, a diamond on the very center or her forehead near her eyes. In this form, Michelle wears silver sandal heels. Her hair had white streaks in it. Two streaks of hair that are about 3 inches wide are curled and pinned at the center of the back of her head, towards the nape of her neck. Michelle is also very good with arrows.
Personality: Sweet and innocent. Very loving. Dispite how it may seem like an act, Michelle's personality is pure. Can be a bit protective at times, but only for the better.
Aliases: Lena Yue"
Doesn't that character sound kinda annoying?