Aliens

Two classes are available upon joining the alien team: the Dretch and the Granger. As you gain kills, you may use your earned evolution points (evos) to evolve into higher classes with the Use Structure/Evolve button. The alien team is mostly limited to melee attacks and must use stealth and speed to defeat the longer range humans. All aliens automatically regenerate health at a slow rate.
The aliens have the advantage of high mobility (some can walk on walls and ceilings) and stealth. The smaller aliens can squeeze into spaces a human could only dream of, and from there they can mount an attack or merely observe.