The Alien is a parasitic creature that has infected one of the USI crew members in the sector. If you are selected to be the alien you will be told at the start of the game. The alien's goal is to kill or infect all other players in the game through any means nescessary. The alien can also evolve twice throughout the game, having three selections each time. This results in 9 final form aliens.
Evolution 1 - Strength, Energy, Manipulation
Strength Evolution - Regeneration, Basilisk, Webspinner
Energy Evolution - Combustion, Lightning, Illusion
Manipulation Evolution - Masquerader, Temporal, Spacial
The alien is generally able to take an unprepared human in a one on one fight. The earliest opportunity to secure a kill or spawn is the moment the game starts, your alien form gives you more health and DPS than the average suit and the infection can be devastating without any antibodies. This is an extremely risky strategy, especially on well populated stations such as the USI Niffy.
The alien's model is actually the hydralisk with a xenomoprh skin from Aliens on it.