To unlock the fifth soldier class (Psi Operative) in the game , you will have to do several things - but as you can imagine, it's well worth it, as the class gives access to a number of extremely powerful abilities, which will vastly increase the effectiveness of your strike team.
To unlock the Psi Operative, you will need to:
- Defeat a Sectoid. This step is quite an easy one, as the Sectoid will be your regular enemy starting from the first mission (not the tutorial one), so you can't really avoid or miss the opportunity to kill him.
- Do the "Sectoid Autopsy" research - after a couple of days you will unlock several new things, such as the new research, "Psionics".
- Do the "Psionics" research - similarly to the above one, completing this research will give you access to new things, as well as unlock the Psi Lab facility.
- Construct the Psi Lab facility - it's quite costly and time-consuming, as you will have to clear the rubble first.
When you are done with the above steps, place a new rookie soldier into the Psi Lab - after several days he / she will be "transformed" into the Psi Operative class.
However, the function of the facility does not end here, as the Psi Operative cannot gain experience points like other soldiers do (by killing enemies). To advance him / her on higher levels and unlock new abilities, you will need to place the Psi Operative back into the Psi Lab to perform further training.
And here comes another novelty - the possibility to select one of the three abilities that the soldier will receive at the end of the training session. Abilities are picked randomly from the whole pool, meaning that, provided you are lucky, you will gain access to the most powerful ones at the very beginning. To speed up the process of creating new Psi Operatives, or training existing ones, you should place an Engineer inside the facility, as well as upgrade it further in the future, so that you can perform two actions at the same time.