Lo he preguntado en un Reddit de México y me han dado distintas respuestas, todas coincidentes en señalar que esa "N" equivale a "No Nombre" y es usada para indicar una inicial supuesta, no la real.

It's used to refer legally to a person who's last name is unknown, or whose last name may not be revealed for legal reasons.

It's similar to "John Doe" in that sense, which in Spanish is "N. N." (Meaning "No Nombre")

Because it denotes that that letter is a supposed name instead of a real one.

Because it is some kind of pseudonym to preserve the identity of said person. It is "N" to emphasize the fact that it is not a real name. If it didn't have the quotes, it could mean that his/her last name actually starts with N.