ADVENT Officer (XCOM 2)
The ADVENT Officer can be seen leading units of ADVENT Troopers.
ADVENT Officers are more durable than regular ADVENT Troopers, and they cannot be killed by a single assault rifle burst. Their Magnetic Rifle deals 3-4 damage, and they have a grenade to deal 2-3 damage shredding 1 armor.
Officers have the ability to Mark a target, making it easier to hit and encouraging all ADVENT units to focus fire on the target. This doesn't end their turn, so they can also fire at said target.
ADVENT Officers are mostly notable as the first target you're required to Skulljack to progress the storyline. Note that skulljacking the Officer will cause a powerful enemy, the Codex, to spawn, so don't do it while in the middle of a firefight. Skullmining an officer after the initial Skulljack, however, will not spawn another Codex.
Their autopsy is required to advance the plot, and reveals that the Officer have an implant similar to the Commander's. It is linked to the alien Psionic Network, and sends the Commander's tactical data to the Officers, who then order the Troopers around. However, due to the Commander's rescue and removal from the Psionic Network, the data has decayed, causing the Officers to make occasionally make silly tactical errors.
The Proving Grounds are unlocked from this autopsy, and the new objective is to build the Skulljack and to Skulljack an Officer, as detailed above.