Mr. X is the male version of the letter "X" in the English alphabet. Mrs. or Ms. X is the female version. Since they have no genitalia it is hard to differentiate between the two.
Mr x is a person who is a threat to the one direction fandom they leave really strange messages and death threats towards the boys. They have been inactive since around 2013 but we still should look out for them.
We need more guards theres been news about mr. X being at a show in 2021