[ Added 05/24/21 ]
The “Egg” is not a living creature, but a unique Quell. One of the indigenous people contained their own essence within this “Egg”, with the intention of returning to life sometime in the future. The Egg began its revival after being discovered by Henry Knights in the Desert Megalith. This was just before Gustave turned the whole world on its head. Meanwhile, the Egg was engaged in a prolonged deadly conflict with four generations of the Knights family. It was finally destroyed by Virginia Knights, granddaughter of Tycoon William Knights, and her companions.
One of the indigenous people tried to resurrect themselves from the Egg, but this was always meant to be a revival of their essence, not their physical body. They sealed their consciousness inside the Egg, with the Egg itself only acting as a container. Therefore, the incidents caused by the Egg during the game were not the desires of the Quell, but of the person whose consciousness was contained within the Egg.
The Egg wanted to absorb large amounts of Anima to use as energy for its revival. Specifically, the Egg wanted the spiritual Anima contained within humans. This spiritual Anima is more powerful and of better quality than the corporeal Anima which keeps plants and animals alive. Since the plan was to revive someone’s consciousness, it seems only natural to use the same type of spiritual Anima.
People who carried the Egg had their own consciousness overtaken. In this state, the person’s own personality is lost, but their memories and basic nature are left intact. Their talent for casting Spells is also greatly increased, allowing them to create effects way beyond what is possible for normal humans. The person holding the Egg carries out its objectives, preparing for the Egg’s revival through methods most suited to their own abilities.
The characteristics of the person holding the Egg had a great influence on their actions. For example, Misty used her feminine charms to steal Anima from the residents of North Gate and Fake Gustave established the Edelritters and led a rebel army in a war against the governmental forces. The Egg doesn’t seek out hosts based on their gender or abilities, but thinks about how best to utilize the abilities of the current host. There are still people that the Egg can and can’t use, of course. But considering the incidents caused by the Egg in the game, maybe the Egg knew who was going to come along and pick it up?
In [The Last Megalith], Ginny and company confront a version of the Egg Quell that was transformed after acquiring Anima, but this doesn’t mean it was the true form of the original person contained within the Egg. However, the Quell itself can be considered a “body” that was created to house the essence of the original indigenous person sealed within the Egg.
The Egg takes on various forms during the fight. This happens because Sargon and the Edelritters are supplying the Egg with powerful Anima. The Last Megalith, which possesses Star Anima, functions as a radio tower so the Edelritters can absorb Anima from the rest of the world to supply the Egg. As a result, the Egg changes forms and releases that Anima as powerful attacks using Flame, Water, and the like.