Soldier in XCOM 2 is the base unit that comprises your squads.
You may recruit new soldiers through the Armory for a cost of 25 Supplies or as rewards for completing some Missions and objectives. Soldiers can be created in the character creator to add to your character pool and will appear as recruits throughout the game's progress. Soldiers enter as rookies and can be promoted to Soldier Classes through combat experience or by training them in the Guerrilla Tactics School.
Soldiers can be fully customized aesthetically as well as their Weapons and Armor. Their Stats may be improved through adding Personal Combat Sims.
Once a soldier dies in combat, they are lost forever and the names of the fallen can be viewed in the Memorial. Injured soldiers who survive missions can be returned to health by a stay at the Advanced Warfare Center.
New recruits start out with the rank of Squaddie. As they gain experience through kills they advance in rank and unlock new abilities per their class. The following table lists the ranks and the rough* number of kills required for each based on the difficulty.
* Soldiers also gain experience through kill assists, but this is not tracked in a way that you can easily monitor. As a result, a Soldier may get promoted from Sergeant to Lieutenant after only 20 kills, for example.