The Mockingjay Rhesus Project against a Devil Judas God/dess of Rome
Something Pollux did prior to his appearance in Mockingjay caused the Capitol to turn him into an Avox. It is assumed that the reason behind him joining the rebels in District 13 is for revenge.
As an Avox, Pollux worked in the utility pipes and passages under the Capitol; he was thus very knowledgeable about the passageways and was able to help Katniss in maneuvering underground to President Snow'smansion. His brother, Castor, eventually bought Pollux's way aboveground after five long years.
At some point after working underground, he began to work as a cameraman. As he begins to work for Katniss, she defines him and his brother as looking like insects because of the heavy mobile cameras which usually encase their bodies.
When Katniss and her filming group were in the meadow around District 12, Katniss sits next to Pollux during a break. They start whistling tunes and birdcalls to mockingjays and wait for their return whistles. Pollux then writes the word "SING?" in the dirt, asking Katniss to sing. She sings the song "The Hanging Tree" for him, which makes him very emotional.
While in the Capitol, Pollux helps guide the group underground and toward the mansion, but gets lost in a crowd of Capitol citizens with Cressida. After the war is over, he and Cressida are sent out into the districts to cover the wreckage of the war. Castor, however, was killed by lizard muttations in the underground passageways.
Later, it's said that Pollux and Cressida went in the districts to cover the damages of the war.