Canon Sue

A character in fan fiction (typically the author themselves) who is inserted into an existing universe and is unrealistically perfect. A fan-fiction version of Mary Sue: A term for a character who is unrealistically perfect without any flaws in (original) novels (Male of Mary Sue: Gary Stu)

"When she wrote herself into Harry Potter as the best witch ever, becoming the headmistress of Hogwarts while being a student, defeating Voldemort with itching powder and a single sponge, she created a canon sue."