A lot of people say that the “restrainer” is the Holy Spirit, Himself, but if you look at the last 2000 years of history, it looks to me as if the “restrainer” has been the called out ones who are filled with the Holy Spirit, i.e., the Invisible Church. When the Netchatef, aka Harpazo, aka Rapture happens and the Invisible Church is taken, the “restrainer” will be gone, but the Holy Spirit will remain to lead those who are left behind to Yeshua HaMashiach (aka Jesus Christ).
This difference is somewhat foreshadowed in the harvest times of Israel. The Barley is harvested by waving the stalks and the breeze separating the seeds from the stalk. The wheat harvest however required the 'tribulum' crushing of the crop to press the seeds out from the husks. The Barley would correspond to the Church Age of believing and being indwelt, and the wheat would represent the pressing the harvest to get the seed/fruit. The very last is the grape press, to crush the grapes out to yield the juices.