I often see a lot of confusion in modern paganism between three related but distinct concepts: the co-walker, fetch, and fylgja. These three concepts come from different cultures and can be described by some contemporary writers as equivalents however when we look more closely at the concepts within the root cultures it becomes clear that they are not so much equivalents as loosely similar concepts.
The Co-walker - This concept comes to us from the writings of rev. Robert Kirk who is clear that the Co-walker is a type of fairy being attached to but separate from a human being. Kirk describes the Co-walker as looking identical to a living human and being seen by other humans both during the lifetime of the person they are attached to as well as after the human dies although they eventually return to their own people (Kirk & Lang, 1893). While Kirk doesn't describe the Co-walker as being dangerous, or indeed as doing much more than occasionally being seen by other humans as an omen that the living human would be arriving at that location soon, he does make it clear that people with the Second Sight abstained from eating meat at funerals or banquets to avoid sharing a meal with a Co-walker (Kirk & Lang, 1893). Kirk says that people who are able to see such spirits and distinguish them from living humans saw them among the pallbearers carrying the casket at funerals as well as eating at funerals and feasts, implying perhaps that such spirits used their form to move unnoticed among humans. Kirk himself had no idea why the Co-walkers chose to attach to humans saying "It accompanied that person so long and frequently, for ends best known to itself, whether to guard him from the secret assaults of some of its own folks, or only as a sportful mimicry to counterfeit all his actions." (Kirk & Lang, 1893, pages 43 -44 language updated by me)
Fetch - A concept in England that is rather obscure in nature the Fetch in folklore is a copy or duplicate of a person which appears as an ill omen, usually of death (Briggs, 1976). Also called a wraith or double the Fetch would be seen by the living person or those who knew them, generally right before they died (Harper, 2018). In more recent material the Fetch has been given many of the qualities and abilities of the Fylgja, although in older folklore it is clear that the Fetch or wraith was only viewed as a death omen.
Fylgja - A Norse concept, a fylgja may be an independent protective spirit or a projected part of the person's own soul; when it is the person's own soul it usually takes an animal form. Fylgja can follow family lines and there are examples in Norse myth, such as in Hallfraedarsaga, of Fylgja who were inherited through generations or seemed to be primarily attached to one individual but would also aid family members (Gundarsson, 2007). In modern books Fylgja are often compared to or equated to Fetches, but they lack any sense of ill-omen; the Fylgja was viewed as positive and seen as both protective and luck-bearing. It was common for a person's Fylgja to be of the opposite gender although we should note that in tales this occurs most often with men having female Fylgja and sexual elements or relationships were not uncommon between a man and his fylgja-woman. Fylgja may mean 'follower' or 'following' and they can act in decisive ways to aid the human they are connected to, providing knowledge as well as physical protection (Gundarsson, 2007). Claude Lecouteux strongly connects the Norse concept of the Fylgja to fairies, arguing that Celtic examples of fairy women who act as tutelary spirits and protectors of family lines as well as those who attach themselves to individual humans are the same beings that the Norse would label as Fylgjas (Lecouteux, 1992). He refers to these spirits as 'Doubles' and points out their many similar characteristics and functions to Fylgja.
It is understandable why there is such confusion between these terms, especially as all three are sometimes called 'doubles' in English. I have myself used and written about the term Fetch in a more Fylgja sense based off what was written in the book Our Troth volume 1 (generally a good source) something that I am now less comfortable using. The more I've researched it the more I've found a clear association with the Fetch as a death-omen rather than a helper spirit. Similarly a Co-walker is clearly not a Fetch - Kirk writes about those under the name wraiths later in the same section of his book - and does not fit the description of a Fylgja. I would also note, to avoid further confusion, that these spirits are not what we would term Familiar spirits either, as the Fylgja either attaches to family lines or a person at or before birth, the Fetch is a double of a living person, and the Co-Walker duplicates a living person for its own obscure reasons while the Familiar spirit is given to or chooses a person later in life and acts as a mediator and aid in their magical and spiritual work. I think for myself I might start using the term 'Follower' to describe in English the type of guardian/guide spirit that we see in some folklore and stories and which fits the category of the Fylgja to avoid this confusion of terms.
Gundarsson, K., (2007) Elves, Wights, and Trolls
Briggs, K., (1967) A Dictionary of Fairies
Kirk, R., and Lang A., (1893) The Secret Commonwealth of Elves, Fauns, and Fairies
Lecouteux, C., (1992) Witches, Werewolves, and Fairies
Harper, D., (2018) Fetch; Online Etymology Dictionary