Basically you'd press some key on your keyboard to open a menu that let's you name the human NPC you want to spawn and choose a skin for it. Then it will spawn from the world's spawn. Also make a menu to remove an NPC because I want them to respawn when they die.
You can make an NPC teleport to you or teleport to it like in a server. They will say random things (like in Minecraft Comes Alive) and you can ask ask an NPC to follow you. When an NPC follows you, they'll copy what you're doing. For example, if you begin mining, they'll begin mining independently somewhere nearby. If you're fighting monsters, they will too.
All monsters will attack these NPCs as if they were players. If an NPC dies, it re-spawns at spawn and you also see a death message describing how it died, like in a server too.
Basically, it;s like having pretend friends in single player