Technically, that's an "Author Insert" character. It's usually a Mary Sue, especially when it comes to fanfiction (or fanfiction inspired published works), but it isn't always. Much of the rest of WW's definition is sound, but also the following:WhiteFangofWar said:Seconded.WinterWyvern said:Really now, it's simple.... Mary Sue characters are a self-insert of the author, are usually invented by 13 years old girls, and are absolutely idealized, and every single thing in the universe revolves around them.
◘ Secondary Characters perform the work of the narrative, but main character is credited with the successes
◘ Never changes; instead the story or other characters change to solve any dilemmas the Mary Sue encounters
◘ Is often nebulous in longform stories; adding details to their character that are otherwise known to be "impossible" in the mythology of the story-universe
::On the Original Topic::
Honestly, I'd say Clara Oswald from Doctor Who qualifies as a Mary Sue. Not wanting to spoil the show for those who haven't seen it yet, but the entirety of season 9 exists to remind us of how awesome she is; including episodes she's not in, where secondary characters talk with the Doctor about how magnificent Clara is.