I think Posca's a brilliant character. It's curious that we know so much about Tiro, Cicero's secretary, and nothing about the equivalent man/men who must have worked for Caesar.

He's clearly referred to as a Greek (as one might expect) by Antony, but otherwise we don't know much about him. Am definitely looking forward to him playing an important role as the series closes. If he dies, I shall be very upset!