He was an archeologist, who took part in a Weyland Industries expedition to Bouvetøya, Antarctica in 2004 to investigate an ancient Pyramid buried beneath the ice there. The Pyramid was discovered to be a Yautja temple and hunting ground where the Predators would breed Xenomorphs for use in their Hunts and the expedition soon became caught up in the battle between the two species.
Soon Alex and Sebastian were the only ones alive. During the fight Sebastian was able to translate and reveal more about the Yautja's history with Earth including the pyramid's use in their initiation hunting rituals by reading the hyroglyphs inside the temple. Armed with this new knowledge, Alex convinced Sebastian that they should side with the Predators and give them the Plasma weapons they casually had taken from them, reasoning that once they had them, the creatures would turn their attention to killing the Xenomorphs and humanity would be saved from being wiped out by them.
He was later captured by an alien and then impregnated. Alex mercy-killed him before it could hatch.