Terror on the Moon

John Walker: He is dead at the start of the mission. He has been replaced by a mimic. This strange creature pretends to be your companion and wants you to take him back to Earth.
Peter Stew: Died at the hands of the mimic too
George Stevenson: Head of the expedition. He refuses to return until he finds out what the Soviets were looking for here.
Facundo Gomez: Mexican, loves tacos and burritos, and always carries a knife even on the moon. He realizes there is something different about John Walker but he doesn't know what. He is clever and perceptive.

The Russo-Chinese were here to discover the true nature of the moon. They found that it was a space station belonging to a now extinct race, beneath the rocky surface there are unknown facilities. The mimic killed them. But before that, they sabotaged their own ship so that the mimic could not return with it to Earth.

The corpses of the real John Walker and Peter Stew are just a few meters from the destroyed Soviet ship

Alien of an unknown species that once inhabited the moon. He wants your ship to get out of there. He replaced John Walker. He is tremendously cunning and intelligent. He has other friends beneath the lunar surface that still sleep.
He is stronger than a normal human and can breathe without an astronaut suit.