The Masked Man is a recurring character whose main characteristic is the use of The Mask. His intentions are unknown, not being clear if he wants to help Jay and hurt Alex Kralie or if he is just trying to use one or both of them. Some believe he is totheark because he is seen in a few of his videos. He's called "Masky" by the fandom.
Far into the series, we learn that there are actually multiple masked people. The second most prominent masked person is the Hooded Man, and they seem to be working together.
Warning: Major plot twists and reveals are below, do not read the entry appearance description if you have not yet seen it.
As shown in Entry 35 the Masked Man is without a doubt, or one of them and he is executed in front of J… but the guys doing it looked like pros, could there be more of these situations? (Note: this theory has been proven false in Entry 36. Jay explicitly states the Alex broke Masked Man's leg then left him.)
In many entries it can be seen that video and audio distortion as well as amnesia occurs around the Masked man in a similar fashion to encounters to The Operator.
Possible reasons for this are:
The Masked Man has powers similar to The Operator
(how this occurs can have any number of theories)
The Operator is always present but hidden wherever Masky is
People who have had prolonged exposure to the effects of the The Operator's influence may begin to exhibit lesser effect to those around him/her
(Many theories state that the Masked man has had considerable contact with T.O. and may have become 'Electro-statically charged' in a sense.)