Who is Polemarchus from Great Dialogues and what is their importance?

Polemarchus is Cephalus' father. He is also a member of the audience for "The Republic." After Cephalus and Socrates speak, Polemarchus suggests that justice is doing good to one's friend and evil to one's enemy, a definition which Socrates verbally negates. Polemarchus' passivity in the argument is the reason that Thrasymachus enters the conversation.