Castor and Pollux are twins who serve as cameramen for Squad 451, the rebels. Pollux is an Avox, or a mute; he lost his voice as punishment for a crime and had to serve as a mute servant in the Capitol for years.
We see his pain and joy when he cries as Katniss and the mockingjays sing to each other during a propo. If Katniss is, symbolically, a mockingjay, Pollux is symbolic of another creature that has been cruelly manipulated by the Capitol and left to deal with his losses on his own.
Pollux ends up proving invaluable to the rebels: he did a lot of service in the tunnels underneath the Capitol before, and therefore knows his way around a super treacherous area. Without him, Katniss and the others would have had no chance of infiltrating the Capitol.
Castor dies during the final battles as Squad 451 tries to make it to President Snow's palace. Pollux appears to have made it to the end.