For those who haven't heard of the following term, a Mary Sue is a word often used to critique a character in a piece of fiction who, to put simply, is TOO perfect.
What do I mean? Well lets take One Piece for example. Imagine if Oda introduced a character who's Devil Fruit ability was being a water Logia. This non-existent character would be SUCH an Achilles Heel to over half the OP cast because a majority of Devil Fruit users have a standard weakness where if half of their body is submerged in water, they cannot move. Now also imagine if this character was some how immune to Haki (for those who don't know, there is a certain ability called Haki which has two variations: one that can instantly KO fodder enemies by realising a wave of energy, and another in which you can imbue your hands/legs or weapons to enhance attacks; so much so that you can even harm characters who possess elemental intangiblity (A.K.A. Logias)). So I ask again, can you imagine if this character was immune to that? Pretty friggin' cheap huh? What if I also said this character had Batman level intellect? Aaaaand just for the heck lets just say this character managed to defeat Luffy, Zoro AND Sanji at the same time... with one hand... while blindfolded.
THAT is a Mary Sue. Now granted, there are some who are still afflicted with this case but not as much as someone similar to the character stated above. For example, Mikasa Ackerman (from Attack On Titan) is insanly skilled and can easily kill off enemies without so much as neither a sweat or scratch. HOWEVER, she has a weakness of being completely devoted to the main protagonist: Eren Jaeger, and, quite honestly, aside from her skill she presents no ground breaking abilities or ones that are just too good for the story to remain interesting (like, she can shoot lasers that kill Titans in one hit and can't regenerate from it).
Anyways getting back the main purpose of this thread, I ask: do you know any characters you consider to be Mary Sues? If so, why?