Old Souls are people - specifically humans - that have been reincarnated through many lives.
Old Souls tend to follow a similar cycle in each of their lives, eliciting deja vu, though they rarely remember any of their previous incarnations. They do not return to settle old debts or right wrongs, as spirits often do. Old Souls are usually very tolerant and wise, as they have experienced lives as people of other races, creeds, genders, religions, etc. Whether or not they perfect a life, they will still reincarnate. The damage an Old Soul has endured during its last lifetime determines when the soul will next awaken. The more damage he or she endured, the more time that passes between lifetimes.
When an Old Soul returns to Earth, he or she may have characteristics of the person they were in their original life. The new body may have a similar name or hairstyle or hobbies, which helps make it recognizable to Night People that have lived for centuries.
The essence of an Old Soul is always the same: the spirit does not change. As a result, they will always have the same soulmate - it's implied that soulmates, after they have found each other, may be reborn together and thus spend many lives with each other. It's also not unusual for Old Souls to have Old Soul friends who reincarnate with them. Many times the relationship between these spirits is preserved, though karmic shifts can cause a mother and daughter to reincarnate as father and son or teacher and pupil, and so on. Although it is assumed that Old Souls comprise a small portion of the human population, it is impossible to know how many Old Souls exist.
Awakened Old Souls Edit
Some Old Souls can recall some aspects or details of their past lives. In most cases, this may come as a sense of deja vu or in dreams, but more rarely, there are "awakened" Old Souls, who can vividly recall their past lives. Hugh Davis is an example of an awakened Old Soul - in his current incarnation, he remembered how to kill a werewolf after witnessing it in a previous life. Hannah Snow is another example - she is able to recall some basic fighting skills from a previous life as a warrior and how to make crude stone tools and weapons from her life as a cave-dwelling girl in prehistoric times. She even unknowingly writes herself notes leading up to her seventeenth birthday, attempting to warn herself of the coming danger (as in each of her previous lives, Hannah was killed before she turned seventeen). Old Souls can apparently access and re-live their previous incarnations via hypnotherapy.
Other Species Edit
So far, the only Old Souls to appear in the series are all human. It's unknown if other species can also reincarnate - given that the witches know of Old Souls and are quite similar to humans, it's possible some witches can be Old Souls, although when Thierry Descouedres is describing reincarnation to Hannah, he specifically describes Old Souls as being "humans that keep coming back". It is known, however, that vampires cannot reincarnate, even if they are a made vampire who was originally an Old Soul. It is unclear why this is; Maya suggests it's because vampires have no souls, though the fact that vampires can be affected by the Soulmate Principle makes this doubtful.