When each character in His Dark Materials is introduced, they are accompanied by a talking animal known as a daemon. For instance, the pilot's opening scene featured Lord Asriel ( James McAvoy ) and his snow leopard Stelmaria (voiced by Helen McCrory). In this fictional world, the souls of human beings live outside their bodies in their own physical forms, walking alongside them and often offering counsel.
Children's daemons take multiple animal forms, often shifting to adapt to their environment, before assuming a single fixed body, or "settling", as the child enters adulthood. The shape a daemon takes as they come of age is different for each person, and often reflects something about their character.
For example, while Mrs. Coulter (Ruth Wilson) is kind and charming on the surface, her daemon, an incredibly menacing-looking golden monkey , suggests she has far more sinister intentions than it might first appear.
It is also heavily implied that a daemon's final form will influence the direction a person's life will take: if your daemon becomes a creature that needs to live in or around water, you are all-but destined to become a sailor or fisherman. Witches, meanwhile, always have bird daemons who can fly alongside them. (In the books, staff have dog daemons, indicating pretty classist, predetermined notions of the master-and-servant dynamic).
The original novels adapted for His Dark Materials contain plenty of daemon lore, which is already making its way into the show. For one, a daemon can never stray too far from their human; staying apart for too long causes extreme pain and distress. Also, daemons tend to be the opposite gender to their human. While daemons might occasionally fight each other on behalf of their humans, it is a strict taboo to touch somebody else's daemon.
Additionally, a daemon may contain your spirit, but that doesn't mean it will always necessarily agree with you. For example, when protagonist Lyra (Dafne Keen) is especially willful, her daemon Pantalaimon, Pan for short, urges caution, almost like the super ego to her ego. And perhaps most importantly, if you die, so does your daemon.
The relationship between a person and their daemon, especially a young person and their daemon, will become a key plot point as the first season of His Dark Materials progresses.