Definition of Mary Sue in English:

Mary Sue

noun

  • (originally in fan fiction) a type of female character who is depicted as unrealistically lacking in flaws or weaknesses.

    ‘she was not a ‘strong woman’ so much as an insufferable Mary Sue’
    ‘she has no flaws, and if someone called her a Mary Sue, I'd have a difficult time arguing’
    as modifier ‘these novels are high on Mary Sue characters’

Origin

1970s: from the name of a character in "A Trekkie's Tale" (1973), a parody fan fiction story based on the US television series "Star Trek".