The Psi Operative is without a doubt XCOM 2's flashiest class, and it may very well be its most powerful. Although unavailable from the start, unlocking the game's fifth class doesn't take too much work. Below is how to unlock the Psi Operative.
1. Kill a Sectoid and gather its corpse.
2. Head to the Research Lab in your base and perform an autopsy on the Sectoid Corpse.
3. Research Psionics in your Research Laboratory.
4. Build a Psi Lab.
5. At the Psi Lab choose which recruit you would like to transform into a Psi Operative.
The Psi Operative is the game's debuffer class, capable of inflicting devastating afflictions to foes, making it the best choice for crowd control. It has great range and can deliver huge burst damage consistently, especially when equipped with Void Rift. It's also the only class that randomly learns skills, rather than through traditional means.
You can check out the Psi Operative's skill tree here.