They're called Heartless. They appear when darkness in your heart completely takes over. And they steal the hearts of others... for some reason. Those spawn Nobodies, which are heartless beings devoid of emotion. Some of them are humanoid and those humanoids comprise the Organization, which is the group in control of the city. There are also Lumens, but I haven't had enough time to study them.

As for the people in City hall, it seems to change a lot. There's this Maleficent person there, though.