Volunteer (EU2012)

Volunteer about to enter the Gollop Chamber

The Volunteer is the XCOM soldier chosen to interact with the Psi Link after it is researched and the Gollop Chamber is built. The Volunteer can be chosen from any Psionic soldiers currently available on the Barracks but you'll need to equip it with a set of Psi Armor, after the Gollop Chamber is built.

Volunteer on the Temple Ship's bridge

After the Volunteer uses the Psi Link, the final mission will become available but the player won't be able to complete any more Research, although it will still be able to build items to undertake the Temple Ship assault. The Volunteer will immediately progress to maximum Psionic rank, so the player will be able to finish leveling up the Volunteer's Psionic abilities on the mission loadout screen if not already a Psi Operative.

During the mission the Volunteer must be kept alive during the entire mission until its end, where it will sacrifice itself to stop the alien invasion and save mankind.