Souls by themselves are described as shining, like a living ribbon, with around one thousand feathery attachments. Wanderer has 1008 of these attachments. It is unknown if this number differs between Souls. Their chemical make-up is very different from humans and human poisons don't affect them.When outside of a host body a Soul is mostly helpless and will succumb if not placed in hybernation inside a cryotank. It is known that outside of the exception of the cryotank, Souls will succumb very quickly to too cold or too hot environments, as seen when Wanderer kept another Soul from freezing to death by keeping it in her Bear host's pouch. The Origin is the only place where Souls can exist for long periods of time outside of a cryotank or a host.
In between hosts, Souls hibernate in cryogenic tanks in a state of deep suspended animation. When an appropriate host is found by The Seekers, the Souls are surgically inserted by their own medical specialists through an incision at the top of the host's spinal column.
In terms of reproduction: One in about five thousand of mostly sexless, sterile Souls has the ability to reproduce. The Soul makes a conscious decision to reproduce, which sets a process in it's body in motion. The body apparently rips itself apart while each cell turns into an infant Soul. About a million Souls are produced in this way, while the "Mother" perishes. The infant Soul has a portion of it's "mother's" memories (who in turn has some of it's "mother's" memories). In this way Souls remember certain events that happened to an ancestor. Sometimes Souls, who are hosted in male and female humans, can have sexual interaction which in turn can fertilize a human egg, and a soul can be implanted as soon as the baby is born. Whatever human consciousness was there is erased. Infants occupied by souls are described by Wanderer as "Miniature adults'.
Souls do not age (while their host bodies may) and can apparently live indefinitely as long as a host or cryotank is available.
Souls have their host bodies eat whatever the body considers as food. The Soul itself doesn't feed but may perhaps gain nutrients from the host body.
A small minority of Souls called intellects have a different physical appearance, they are usually about two or more times bigger than an average Soul due to the intellects' large brain capacity. When intellects are in hosts like humans they must take a special drug which helps the host body cope with the large amount of energy needed to maintain the Soul. Intellects can ultimately suppress a human indefinitely, even a host like Melanie Stryder.