The term ‘Fylgja’ literally means “follower” (“one who follows”). In fact, “Fylgja” is a term that later Europeans coined for a witch’s “familiar” (or a “fetch”). A Fylgja can take either an animal, or a human shape (if human, it will usually show up as a pretty young woman, or a crone). Sometimes the Fylgja belongs to an individual, and sometimes it belongs to the ancestral line (a “kin-fetch”); if so, the shape that it takes represents that family. Fylgjur were considered rare back in Old Norse times; usually only a hero, king, or a vǫlur, seiðkonur and vísendakona seemed to have one version or the other.
The Norse cultures of the Viking age understood the non-corporeal part of a human being to be very complex. A person had a fylgja (fetch); ørlǫg (tally of deeds; the foundation upon which one’s wyrd, or fate, is based); and hamingja (female guardian angel that decided person’s luck) are just a few of the many parts that make up the human soul. Because the roles and definitions of the Fylgja and related concepts of Dís (female ancestor spirit) and hamingja often overlapped and changed over the years, however, it is difficult to give one coherent definition of what a Fylgja is and what one can do.
That said, Fylgjur, as the people in the Old Norse cultures would have recognized them, are still actively showing up in many people’s spiritual practices and lives.
A Fylgja usually carried out several roles dependent on the form that it takes. If it was animal-shaped, it is the shape that one’s soul takes when he/she has left her body, for example when doing any kind of shamanic-type journeying. Accounts in the lore tell of a person’s Fylgja showing up in the real world in their animal form far from where the person’s body actually is, indicating that the Fylgia’s human was trying to send a message or help out in some way. The Fylgja appear in dreams, as well.
A Fylgja can also take a human shape. One way a human Fylgja is represented is as a protector, of either a specific hero, or of his entire family line. The other role a Fylgja can take is to warn a person of their approaching death or prophesying the deaths of those in the family.
Maria Kvilhaug says:
"A woman Fylgja is a female supernatural entity who acts as a guardian spirit for the clan, and especially for the chief of the clan. They were also attached to individuals, but were immortal and appear to have been attached to particular lineages following a person from each generation. Mundal believes that they represent the spirits of ancestral mothers, a part of the ancestral mother worship we know existed among the Vikings. Every human being may have one or more woman Fylgja. Some are visible whereas others are invisible. Of the visible Fylgjur a person has a limited number (2-3-9), of the invisibles a whole flock. The followers are carriers of an individual`s or the clan`s fortune. The woman follower appears often in dreams but also in visions."
Either way, the human Fylgja appears to a given hero, and he must choose to accept the Fylgja into his life. If he does not accept her (as in the Hallfreðar saga) she must wait for someone else in the family to accept her. Though the Fylgja appears to know a hero’s fate, she does not seem to be able to directly change it, though she can influence the hero to act in one way or another. (In one example from Vatnsdæla Saga, a hero’s Fylgja causes him to get sick and thereby avoid attending an event at which he would have been killed.) Still, there is no actually escaping one’s set fate, in the Norse worldview. Eventually what the Norns have decreed will happen. Still, in all but a handful of instances, Fylgjur — both human and animal — appear to be wise and work on behalf of the human who they are attached to.
In the aspect of an ancestral/family Fylgja, fans of the Harry Potter series will see similarities with the Patronus spell. A Fylgja may well be described as a sentient “Patronus”. Similar to Fylgja, the Patronus spell takes on a shape representative of that individual, or family, protects the one who casts it, and appears to the caster when danger is at hand. Another good example from pop culture, this time of an individual’s Fylgja, are the “dæmons” from Philip Pullman’s Dark Materials series. These are semi-autonomous animal-shaped spirits whose form is directly shaped by the personality of the human with whom they are partnered.
In modern reconstructed Heathen spirituality, a Fylgja can be seen as an attendant female spirit or animal, which may visit you in dreams, or appear if you are practicing Seiðr, trance-working, or going on a spirit journey. People sometimes feel that their Fylgja has run on ahead of them when travelling in a physical sense.
Caution would be advised for the curious who wish to discover these beings. Their powers are well documented in the Icelandic Sagas, telling how they may bestow hamingja or luck upon those who they attend, even helping to shape a person's fate. If you displease them, they can leave you, or the benefits they bring could be reversed.