The designated uniform of an Evangelion pilot. In addition to further protecting the body from physical impacts/shockwaves, the suit acts to raise the pilot's Sync Ratio, allowing better control during combat.
From the feet to the neck, the Plug Suit is one-piece and has a structure similar to that of a wetsuit. It fits rather loosely on the body until a switch is activated on the wrist, causing the suit to shrink perfectly against the body of the pilot. The suit also comes with two head pieces, which Asuka Langley Sohryu chooses to wear as decoration.
The Plug Suit, although helpful, is not required for the operation of an Evangelion Unit. Shinji Ikari in particular, whose Sync Ratio is naturally high, has piloted Evangelion Unit-01 on more than one occasion without this suit.
Based on The Red Cross Book