Here's a messed up Headcanon or AU:
What if Calvin absorbed Rory's mind and consciousness when he ate/absorbed/assimilated him because that's easier then regenerating a whole new brain from the single celled organism that it had been reduced to with the extinction of Mars.
That would explain how Calvin seems to instantly know the layout of the station and how everything works. Calvin also seems to become more impulsive after eating Rory whereas before it seemed more slow, cautious, methodical. Testing the waters versus feeling right at home on the station which has suddenly become such an easy hunting ground.
What if "Calvin" fully remembered his life and identity as Rory Adams... but only after killing half the crew...