When you complete the following tasks the corresponding character will become unlocked.
Defeat Acrid in Sunken tombs.
Complete Stage 3 (Sunken Tomb, or Frozen Fundra)
Defeat the Magma Morm, Collosus, and the Wandering Vagrant.
Purchase 40 Drones.
Open HAN-D's container on the USS Contact Light level.
Collect 15 Monster logs
Complete the game five times.
Defeat the Direseeker in the Magma Barracks level.
Complete the game.