A spirit animal is a lifelong partner that is bonded to a human. Spirit animals commonly appear to/are summoned by a human of the age of 11-12. The bonds between spirit animals and humans is preserved by The Evertree. However, in the first Devourer war, the Evertree was damaged and people started to develop the bonding sickness. With the sickness came the gift of the Nectar of Ninani from Ninani the Swan, a substance that is drunk at a Nectar Ceremony to safely summon a spirit animal. After the events of The Evertree, the bonding sickness epidemic ended and the bonds form naturally at any time between 11-12 years of age.
Summoning a spirit animal
Not much is known about spirit animal bonds prior to the first Devourer war. After the war, it was common for people to drink the Nectar of Ninani and call a spirit animal. However, drinking the Nectar did not guarantee a spirit animal, and many people did not have access to the Nectar in the first place. The continent of Stetriol in particular was deprived of the Nectar, causing thousands of people to develop the bonding sickness and die. The Bile was then created from the Jade Serpent talisman to serve as a substitute to the Nectar. The Bile successfully cured the bonding sickness, but it caused the human to be in total control of their spirit animal, and allowed Gerathon the Serpent to take over the human or spirit animal's conscience and control them.
When summoned, a spirit animal is always an adult. It can also be any gender. Their names are decided mutually between the spirit animal and the summoner. (In the Four Fallen's case, they already had names, due to them being Great Beasts.) Usually, people summon spirit animals that are native or common to their area/region. Although there is a vast range of animals that could be summoned, insects, fish and humans cannot be summoned. However, arachnids, such as spiders and scorpions can be summoned, but it's rare.
A spirit animal dying was described as "like losing a limb". In extreme cases, a spirit animal dying causes the summoner to go insane, possibly resulting in death. The same applies if a summoner were to die and the spirit animal live. The spirit animal will become extremely aggressive and unstable.
It is possible to have more than one spirit animal in a lifetime, but never multiple at the same time. The only characters to have multiple spirit animals are Lishay (who bonded to her deceased brother's spirit animal when her own was killed) and Zerif (who had a bile bond that was broken, and later bonded to The Wyrm).