I think the identity of the Emperor is an in-story mystery.
When it comes to signature individuals, Republic side is basically headlined by Satele Shan, even though she isn't (I think) politically in charge of the Republic.
Shes the girl in the last trailer correct? The one that fought against the 2 Sith guys and then gave her master her Double Lightsaber?
And is this the sith emperor?
Or is that in the past?