A badly-written new character in a fan-fiction that upstages the original cast in an offensively unrealistic way.
A TREKKIE'S TALE
By Paula Smith
"Gee, golly, gosh, gloriosky," thought Mary Sue as she stepped on the bridge of the Enterprise. "Here I am, the youngest lieutenant in the fleet - only fifteen and a half years old." Captain Kirk came up to her.
"Oh, Lieutenant, I love you madly. Will you come to bed with me?"
"Captain! I am not that kind of girl!"
"You're right, and I respect you for it. Here, take over the ship for a minute while I go get some coffee for us."
Mr. Spock came onto the bridge. "What are you doing in the command seat, Lieutenant?"
"The Captain told me to."