The Centurion will be encountered early on while the player is exploring the Training Facility. While in the Rearing pool, when player has Billy turn the crank to lift the cage and then has Rebecca pick up the Key, a scene will play where the Centurion reveals itself and captures Rebecca.
The player will be left with Billy to fight the creatures and can not exchange items with Rebecca. The player can not actually harm Rebecca while fighting, and upon defeating Centurion, Rebecca will be freed and it will drop the Key back down.
During the fight, Centurion is relatively non-aggressive. It will simply travel around the room until it happens to come up to Billy where it will then lift the front part of its body and prepare for an attack. Walking into Centurion while it is moving will do very light damage.