One of the characters in-game will directly represent the player. This character will typically be the base leader. If a player's character dies, then the game will be over for that player. In singleplayer they'll be able to load from a save file, but in multiplayer they won't be able to so will have to create a new character.
Action and Architect modes
The player can switch between Action and Architect modes at any time.
In Action mode, the player will directly control the actions of their character. They'll be able to explore freely, recruit, and fight.
In Architect mode, the player won't be tied to any particular character and will be able to issue orders - for example, to build a wall. It works a lot like RimWorld and other basebuilding games. Architect mode only allows you to view and manage your base and areas that you own - whilst the player's character is away, they can use it to check on their base.