Ash
Ash is the science officer on board the 'Nostromo'. What the others of the crew don't know, is that Ash has another mission, a deadly one. The company knew about the xenomorph, even before the 'Nostromo' discovers the signals, and send Ash to collect a specimen and study it. Ash wasn't an old member of the crew, but recently added, so the rest of the crew was unaware about his secrets. One was that of the xenomorph, the other that Ash really was an android, an artificial person. Ash malfunctions and attacks Ripley. Parker helps her, and hits Ash's head off. In the end, they reactivates Ash to question him. When the crew was done, Parker fries Ash with his flame-thrower.