My view of the "Restrainer" is not popular in a number of places, but it was early in Church history. I believe he refers to the Roman political leader in whatever time Antichrist is prevented from appearing. Daniel saw the Roman Empire as the 4th and last Kingdom with significance in this regard.
The 4th Kingdom became European Society, once it had been Christianized. But Daniel said that until this Society broke up into at least 10 kingdoms Antichrist could not gather these parts together to form a new, modern European Empire, patterned after the old Roman Empire.
So it has been European leadership that has prevented the rise of Antichrist up until the present day. But we're getting close to the day where European leadership actually welcomes in Antichristianity as a new imperial leadership.
I know that since Disipensationalism has become accepted in the US the Restrainer has been popularly viewed as the "Holy Spirit." But I see nothing in the text itself that calls for that.
In fact, Paul seems to make reference, privately, to something he had told the Thessalonians earlier. And it likely had to do with pinning the "Antichrist" label on the Romans--something he would not publicly reveal in a letter.
But you decide....