Nacene seemed to be at least partially non-corporeal or energy-based, but after their death a physical component, crystal-like and significantly smaller than the living form, remained. They existed, or were capable of existing, in subspace, and were capable of manipulating it. Their natural appearance was that of a glutinous liquid and reflective cell, about two meters in diameter. They possessed the ability to change their physical appearance at will. Nacene reproduction concerned males and females, using spores, although they seemed to believe that reproduction with humanoids was possible if they possess a compatible biomolecular pattern, but compatible humanoids were rare in the Milky Way Galaxy. Lifespans of at least one thousand years were not uncommon for Nacene. (VOY: "Caretaker", "Cold Fire")
Culture and history
Not much is known of Nacene society, as the only specimens of the species Starfleet encountered were isolated individuals separated from their own kind. Nacene considered themselves a race of explorers, and an expedition reached our galaxy. After the effects of their method of crossing the gap of galaxies accidentally rendered the Delta Quadrant planet Ocampa uninhabitable in the 14th century, they decided to leave two members of their expedition behind, to care for the Ocampans. (VOY: "Caretaker") At some point in the 21st century, one of these "caretakers", as the Ocampa called them, left the other, taking several hundred Ocampa with her, intending to develop their telepathic abilities to a greater extent than the Caretaker was willing to do. (VOY: "Cold Fire") By 2371, the remaining caretaker was dying. He used a displacement wave to bring ships to his array from all over the galaxy, searching for a compatible lifeform to procreate with, to create a successor who could understand the responsibility of caring for the Ocampa. He was not successful, but even in the last moments before his death, he did everything to protect the Ocampa – buying them some time from their enemies to learn to care for themselves, providing them with at least five years' worth of energy, and then sending them information about how to survive on their own. (VOY: "Caretaker")
Nacene were capable of great accomplishments, but it is unclear if these were to be attributed to technology, or merely their sporocystian nature, especially in the case of their subspace manipulation capabilities. They demonstrated a deep understanding and mastery of subspace, by which they were capable of transporting entire ships across tens of thousands of light years in mere moments. It is likely that they used a similar technology to bridge the gap between their galaxy and the Milky Way. They possessed holographic technology, advanced medical technology, and technology to beam large amounts of energy from a space station to a nearby planet. (VOY: "Caretaker")
The String Theory book trilogy states that the Caretaker, Suspiria, and numerous other Nacene were exiles, living away from their people as a punishment for tampering with the cosmic strings that the universe consists of; the other Nacene believe it was a sacred duty to care for the strings, although Q told the crew that the Nacene were exaggerating and the strings would be fine on their own. When Voyager came into possession of the Key that would allow the Nacene to return to Exosia, their home dimension, a Nacene attempted to infiltrate Voyager by altering the crew's memories to make them believe that the Nacene was Janeway's sister, who had been with them on the mission from the beginning. Despite her attempts to escape attention, she was quickly discovered thanks to Naomi Wildman and Harry Kim, whose molecular structures were slightly out of sync with the rest of the crew due to them both coming from the duplicate ship created in "Deadlock", and were thus immune to her tampering (She even activated a backup version of The Doctor to try and escape attention, but the two programs were so different that the crew soon realized that something was wrong).
The Nacene launched an assault on Voyager to regain the Key, but Voyager was able to hold them off, using the neurotoxin that Tuvok had developed in their encounter with Suspiria. Meanwhile, with aid from Q and a returned Kes (along with some subtle manipulation of another Nacene), The Doctor was able to travel into Ocampa's distant past and oversee the birth of a Nacene/Ocampan hybrid, who was able to stabilize reality and negotiate peace between the two Nacene factions (after the hybrid's future self was tracked down by Tom Paris and Harry Kim, aided by a young Q). Aided by Kes, the Nacene evolved to a higher stage of being, as Kes and her "son" – only Kes' body could cope with the strain of giving birth to the hybrid, although another Ocampan contributed the genetic information – returned to Ocampa to repair the ecological damage that the Nacene caused long ago, and, three years later, rain finally fell on Ocampa once more.