One of the Sphere's most dreaded warriors, this muscular beast blasts through adversaries using dual Auto Cannons grafted to both arms. In addition to this advanced weaponry, the Centurion's sheer size and brute strength makes it a truly intimidating obstacle. When Tommy first fights one, it can tear down the pillars in the area with a single blow of its arms and Tommy can also use the force field doors to sever one of its arms so he can gain an Auto Cannon for his own use. It resembles the Cyberdemon from Doom 3 in size and basic shape.
Tommy finds Jen in a strange pod of sorts and she is unable to feel her legs, according to what she says. The pod opens and we find out why; her torso has been surgically grafted to a vaguely reptile-like beast, resulting in a singular creature that is fairly quick, tremendously strong, and uses quite a few weapons against the player. Seeing as Jen is unable to control the beast she is now attached to, Tommy must either kill the beast, and his girlfriend in the process, or be killed himself. When the creature is killed, Jen is still alive, begging Tommy to 'send me (her) to Uneekwa.' Tommy does so, and requests special training from Enisi to take revenge against the Sphere. This creature is spotted a few times before Tommy's fight with it but it featured a male human atop it. When he fights it the second time before fighting the Keeper, it has the male body on it. It is just known as the Creature X in those incarnations.
After seeing fleeting glimpses of an alien creature that is apparently able to control portals by itself, as well as hearing a psychic describing said alien (as well as Tommy) via the aliens' monitoring of an Earth radio call-in show (Coast to Coast AM), Tommy finally comes across the alien in question, seeking vengeance for the recent death of his girlfriend. It is here the player learns that the Keeper's race seeded innumerable worlds across the galaxy with the prerequisite components for biological life so that, when the times arose, the Keeper's race could harvest that which they initially sowed so long ago. Earth was a barren and desolate rock until this process happened. Because of this, the Keepers ultimately do not care about, or even acknowledge, the rights of the beings that evolve on the seeded worlds, for they are merely protein to be harvested and used at their discretion. It is possible that the Keepers are the actual builders of the Sphere, seeing as it performs the very task they set out to initiate.
The first Keeper Tommy fights is able to summon just about every lesser enemy to fight Tommy. Later Keepers are not able to do this, but they share the same characteristics with the initial Keeper; an absolutely gigantic brain, which gives them the ability of telekinesis (with which to shield themselves, as well as hurl balls of energy or physical objects, such as barrels, at Tommy), and a body structure that is unlike any of the other alien races encountered on the Sphere.
At the very end of the game, Tommy finally comes across the being that is in direct control of the Sphere, who is a human woman who was abducted from Earth long ago. She says that, originally, she was just like Tommy; fighting tooth and nail every step of the way to what has become the Mother's den. At the time, the creature in the Sphere's Core was old and tired of being in control, so it made a deal with her: stop fighting and take its place, so that it would be allowed to die, and she and her people will live. Seeing as it was either this or a swift death, she chose to become the Mother. Now she makes the same offer to Tommy.