You create your avatar character, but Divinity: Original Sin 2 allows the recruitment of three more companions for a maximum party size of four. It is recommended to find companions fast, or it will be harder to survive.
Companions are NPCs who can join your party. They have stories and opinions of their own, which they will share with you the longer they stay in your party. They will want to talk to you about certain events in the game, especially when they are particularly enthusiastic about or disappointed in your decision-making. Your decisions made both in these conversations and in other crucial points in your journey will change the relationship between you and the companion.