Identitarians will complain about a Mary Sue or a Magical Negro and fail to see how the terms obscure the underlying truths:
A Mary Sue is a female wish fulfillment character. For people who think primarily in terms of gender, her male counterpart is the Marty Stu or Gary Stu. The original Mary Sue was a character from Star Trek fanfic who got to do everything a heterosexual female Star Trek fan might want to do if she became part of Star Trek. A smart feminist critique would point out that Mary Sue was fundamentally a feminist response to the sexist casting on the '60s. Instead, a Mary Sue became shorthand for a badly done female wish fulfillment character, and now the term's used to suggest any wish fulfillment character is bad.
But adventure stories rely on wish fulfillment characters. If, as some argue, the lead of the new Star Wars movie is a Mary Sue, Luke Skywalker and Han Solo are also Mary Sues. My favorite Mary Sues are Captain America and Robin Hood.
A Magical Negro is a black supporting character, and often one who dies. Which is to say, the Magical Negro is simply a supporting character. The real complaint with the Magical Negro is not that there are black supporting characters like John Coffey in The Green Mile (who falls under another archetype, the savior figure). The real complaint is that there are few lead roles for black actors.
The list of white supporting characters whose role is to help the main character and often to die is very, very long. My favorite Magical Negroes in film are Val Kilmer's Doc Holliday in Tombstone and Chow Yun Fat's Mark in A Better Tomorrow.
ETA: Please Stop Spreading This Nonsense that Rey From Star Wars Is a “Mary Sue”