The main feature of Single Player mode is the "Partner AI." This is a character /several characters that assist the PC and are CPU controlled. Gameplay will change depending on Partner AI interaction.

Your AI Partner performs actions just like the character you control. He/she can move, attack, use items, chat, help other, etc. Of course, when they run out of vitality they will die like any other character...

You must decide how to interact with your AI Partners. Will you help your AI Partner battle against some enemies. Or will you let them take on for the team and cover you while your run away.