Apoc serves in various roles, from running the tracing equipment for ejecting new redpills from the Matrix, to being the main driver for the group. He wrote the Four Horsemen virus. He is frequently partnered with Switch.
He is alluded to the dark angel who brings the end. This is exactly what he did when he gave Neo his first handgun during the Hotel Ambush, marking the start of the end of Neo's 'innocence' as he first took up weapons with the intention to use them against others. Apoc is ruthlessly killed in the Matrix when Cypher pulls Apoc's data probe from his headjack aboard their hovercraft as part of his attempt to return his body to the Matrix power plant.