That’s a libero. They’re like a defensive specialist that’s good at digging and getting down on the floor to save points. They also don’t have to rotate like the other players do.
They definitely have to rotate like the other players. They go clockwise like everyone else.
Liberos usually play back row for middles. Once the libero rotates to the front left position, they swap out with the middle that is in that spot and the libero either comes out of the game because the other middle is serving, or the libero goes and serves in place of the middle that just rotated into the back row. If the libero did come out, they go in for the second middle once that person is done serving.
The libero can swap in for anyone in the back row- that is the uniqueness of the position. They can bounce in and out for any player at any point in the rotation.
You are correct that the libero is a defensive position- they need to be loud and aggressive since they lead the back row and defense. They tell the front row where a set is going in defense, they tell front row where openings are to hit, and they usually take the 2nd touch if the setter had 1st touch or can't get to the ball.
The libero has some restrictions too-
- they can't attack over the net even from the back row, if they contact the ball above the net. That's why they pass the ball over almost all the time.
- they can set the ball with their hands if they are behind the 10' line. If they are in front of the 10' line, they have to platform set the ball for there to be a legal attack by a hitter.
The rules are odd seeming to many, but if you keep in mind that the position was created for defense only, the limitations make more sense.