Not-Seven's species were a specter-like race with no legs, at least four arms, and were able to float in mid-air. They had two large black eyes and a bulbous head. The body was covered in delicate looking blue-gray skin. They were probably a hive-minded species, with the one member seen constantly using the word "we" to describe its actions.
The Human mind doesn't process fast enough to see them. But when a Human is in a perilous situation or is about to die, the mind speeds up and the perception of passing time slows, and this is the only way Humans can see and interact with them.
A member of the species was studying Rudolph Ransom's mind just before he died when the USS Equinox exploded. It isolated his mind by injecting a paralytic, making his mind increase in speed. The being was able to take the form of Seven of Nine due to Ransom using his synaptic stimulator. (VOY - Strange New Worlds 10 short story: "The Fate of Captain Ransom")