The Fylgja more commonly likes to reveal itself in one's dreams whereas it's more ghostly apparitions tend to prefer the waking hours. While the Fylgja may, in one way, be understood as the "Hugir" (soul), it can also describe a Fylgjukona or even a Dis, namely; guardian spirits of the female variety; that may defend their own gender, watch over an individual, or a whole kindred.
Sometimes several Fylgjur may be bound to a person to insure their safe journey in life as well as death, often in the form of the three Norns (the weavers of fate). Thus some people may have stronger Fylgjur, in essence; more fortune or "Hamingja" than others. While a Fylgjur may also embody the collective spirit of one's ancestors, it is still to be understood as its own individual entity. All in all these manifestations were generally described as dark forms until the time of Snorri Sturlsson. In his account of the Olaf Saga (Heimskringla), he presents 9 women in light robes from the south as opposed to the nine black robed Fylgjur of Norse tradition, in a cryptic attempt to illustrate some epic struggle of good and evil over the soul of the unfortunate Thidrandi.
Speaking from my own personal experience, we seem a dynamic union of matter (dark) and energy (light) in a convectional node of the local spacetime continuum. Remove the energy and you have gravity. Gravity is what defines substance, whereas energy is what breathes life (spirit) into it, makes it dynamic. The fylgia has always felt to me like that gravitational vessel that holds the soul spirit. Take away that spirit and it feels like a gravitational vortice. This defines the death fylgja. This is what I experienced from countless people who left this world. The death fylgja collects their residual energies until that channel can finally close. In a way you could say it gathers up the pieces. Take that conversely and, by the same principle, you can manifest an alterego.
