Who is Endius from Tides of War: A Novel of Alcibiades and the Peloponnesian War and what is their importance?

Endius is a high ranking Spartan and Lysander's companion. Over the course of the novel, Endius comes to agree with Alcibiades about the need to unify Greece. He is not, however, influential enough to deter Lysander from his plan to destroy Athens.