Pleroma is a hidden base of the U-TIC Organization, located on a series of asteroids. It is technically not a "planet" -- it is actually a planetoid built by fusing several asteroids together. Its solemn exterior evokes an air of sanctity.
This structure was originally a shrine for a long lost ancient religion: Ormus. In the past, it was a place of worship used by Ormus. It was eventually abandoned and remained uninhabited for centuries. The U-TIC Organization reopened it after the Miltian Conflict.
In Episode I, Margulis uses Pleroma as a prison for the 100-Series Observational Realian, MOMO Mizrahi. An Ormus cathedral is located in the center of the asteroid, which is briefly shown in the game. Pleroma is rendered obsolete when Ziggurat 8 manages to infiltrate it and escape with MOMO.
With the secrecy of the base compromised by the Galaxy Federation, it is abandoned in Episode II and destroyed by Sellers and Heinlein. Pleroma is also mentioned by Sellers as having been the name of the original immigration ship from Earth (Lost Jerusalem). It was named by one of Ormus' Patriarchs.
In Gnostic lore, Pleroma refers to a 'divine light' that exists above (non-spatial) the physical realm, occupied by spiritual beings self-eminating from the pleroma--called the "aeons" (Greek ὁ αἰών, 'forever') or "archeons" (Greek ἄρχων, 'ruler'). Yeshua is an intermediary aeon from the Pleroma who can help humanity recover the knowledge of its divine origins, bringing humanity back into unity with Pleroma. Yeshua's counterpart, Sophia, separated from the Pleroma to form the demiurge, giving birth to the physical realm.
In Jungian psychology, pleroma is both "nothing and everything. It is quite fruitless to think about pleroma. Therein both thinking and being cease, since the eternal and infinite possess no qualities."