The Creature
Evil creature bioengineered to a ridiculous extent by the Infinity Corporation and sent within a meteor to Gaera to essentially take over the planet. The creature, which started off as a simple infant with infant-like thoughts, fed off the telepathy of the group closest so it near its 'birth' (which just so happened to be Astrynax, Enlil, End Reshiki, James, Kyle Broadsky, and Sorune Yohlm) and basically gained years of mental development from that very incident, swiftly growing to the mental state of an adult.
The Creature has the ability to enter the bodies of others (provided that there is a wound in the skin for the Creature to enter through) and control them. This is accomplished by spreading 'eggs' throughout the victim's body and mind, and also allows The Creature to control the victim even after leaving their body. The Creature can also adopt abilities and attacks used against it, and replicate its body (to a limited extent) thus insuring that even if one host body is killed, there is likely another hovering about somewhere.
After attempting to kill those who first contacted it, it followed them to Doma and proceeded to attack the group of characters a number of times, leading to pitched battles where such characters as Astrynax, Derek, End Reshiki, Kelne, and Kyle Broadsky were possessed and put under the being's control. Despite the loss of both Kyle and End Reshiki, Kelne and Astrynax eventually joined together, hunted down the nest of the being, and wiped it out.
While it's still about and has attacked the characters on other worlds, it has yet to stage another battle in Doma, or even on Gaera for that matter. Time will tell if it returns yet again or not.