Ehm, no, he wasn't a captain. Remember the part where pippin asks if he's the Captain of a company and if he (pippin) could join? Beregond laughs and says no, he's just a simple soldier assigned to showing pippin around.
He was the captain of the White Company, Faramir's guard in Ithilllien, but that was during the fourth age. That must have been where I got confused, by thinking he was a captain. But look, I haven't rehearsed my knowledge of the books for some time, now, and the films leave plenty of gaps empty.
But even so, there are other gameplay heores who play a rather small role in the War, like Arwen.