The Overmind arose spontaneously as soon as the third Vuduri PPT transceiver became activated. It is a sentient being that resides within the infinitesimal phase delay of the gravitic communication between all mind-connected Vuduri.
The strength and autonomy of the Overmind is determined by the number of communicants. Its personality is also shaped by the individuals and environment so each samanda (seed) is distinctly different.
On Tabit, there were only 80 Vuduri so the Overmind was present and sentient but more of an organizer than a ruling entity. On Deucado, there were thousands of Vuduri so the Overmind was autonomous and communicated directly with Rome quite often. On Earth, there were half a billion minds so the Overmind was a crushing, overwhelming presence. It was so powerful that many Vuduri operated on what was called "second sight" meaning they did not even attend to their own sensory input. Instead, they allowed the Overmind to allow guide their perception of reality.
Rome was forced to accept second sight when she was little. It was the price she paid for being allowed to keep a kitten, named Skodla. However, Rome developed the ability to segregate, or wall off, a part of her mind. When she matured, the Overmind actually exploited this capability as it gave Rome superior hand-eye coordination. She was selected as the data archivist for the Tabit mission because of her skills.
Because the Overmind of Earth was so powerful, many of the Vuduri did not even bother to think for themselves. Most ordinary Vuduri have no curiousity whatsoever and are intellectually lazy. But because the Overmind is the amalgam of the people comprising it, this had a negative impact, a negative feedback which made the Overmind of Earth intellectually lazy as well. This is why it took it so long to send out the Tabit mission. This was also something that MASAL exploited as he neared the end-game.
The Overmind of Tabit was created for the purpose of research and exploration so it was open and receptive to change. This is why Rei was able to change its nature so fundamentally. It was ready.
The Overmind of Deucado was created to operate as a prison warden and held itself in haughty, high regard. However, it was changed by Rome who was actually the first Vuduri that ever spoke to it directly. Since the Vuduri who made it up were not cruel, it was able to change and evolve into a caring, even loving entity fairly quickly.
The Overmind of Rogal Canduro is fairly flexible and impressed with the stunning beauty of that world. The Vuduri there are more animated, more like normal people and have an appreciation for life and living. This is explored in the novel entitled The Ark Lords.