The Unchosen are human spirits who have not yet receive human bodies. THe reason they have not received bodies is related to Mormon beliefs about the pre-existence, but the possibility of Unchosen spirits does not exist, as far as I know in real Mormon doctrine.
They are called Unchosen because in the battle between Satan and God in the pre-existence, they refused to take sides, not choosing to be born into mortality to be with God but also choosing not to follow Satan. Because they have not chosen Satan, but merely delayed choosing any path, they cannot be cast down to the Earth with Satan as disembodies spirits, but they also cannot be in Heaven [even pre-existent Heaven] with those who have chosen earth-life. So, because God will not take their agency by forcing them to choose Satan or the plan of Salvation, they were given the planet HomeWorld, and temporary shape-shifting bodies until they made up their minds. When/if they choose the plan of salvation they can be born as a human. They then become a Chosen One. They do not remember their time as an Unchosen, any more than anyone else remembers the pre-existence. When/if they choose Satan they lose their shape-shifting bodies. This often happens by them choosing to possess the body of another. It is possible for the to possess the body of another with consent for a short time without driving out the spirit of the body, but it is difficult for them to leave a body once they have possessed it.
A Chosen One can foster an Unchosen spirit with less risk than a normal human, but it is still dangerous. Rae does this to bring the spirit of Emily Scott-Harris to Serai through the dream plane with the help of Quetzal. Emily does this to prevent Wikki's spirit from becoming part of Leviathan until she can conceive a body for WIkki , to prevent Leviathan from gaining an unchosen spirit and thus the ability to command the living stone.The presence of the living stone in Gaia's leviathan body makes it possible for any chosen one to call him or cast him away.
They also have the power to steal the body of a human host, casting out the spirit of the human host. This gains them a human body but it is not their own body so they will not resemble the body spiritually and they are etermally damned...sent to outer darkness with Satan and his followers after they die. Because they are known to have this power they are considered demons and feared on HomeWorld.
these spirits, whether Unchosen, Chosen, or those who have chosen to steal the body of another, retain the ability to command the Living Stone . This gives the Chosen Ones the ability to protect the Unchosen from Gaia/Leviathan. As Long as Gaia never obtains one of these spirits his power is limited. Emily Scott-Harris is a Chosen one [as are Jason Scott-Harris, Rae, and Emily Hawthorn] Emily S-H overcomes Gaia/Leviathan by allowing herself to become part of tis monstrous soul stealing organism[Leviathan], and then taking control of it away from Gaia, and releasing all the souls of all those captured by Gaia/Leviathan. She gives her own life as a human to rescue these souls.