Who is Ser Cleos Frey from A Clash of Kings and what is their importance?

Ser Cleos Frey is a knight, He captured by Robb Stark, who sends him to carry a message to Cersei Lannister..... an offer of peace. Tyrion Lannister receives the message and sends terms of his own back with Ser Cleos. Tyrion also includes four of his people in Cleos' band to free Jaime Lannister from Riverrun without Cleos' knowledge. When the men are discovered trying to break Jaime out, Ser Cleos is also locked up and punished, although he protests his innocence.