The Norse fylgja, the proto-Germanic Fulgijaną - is known in our mythology as ones "guardian angel" or "protective spirit". Let us have a look at the etymology of the word from the Oxford dictionary of Old Icelandic from 1910:
fylgja f (in nominative plural fylgjur) meaning company in the act of accompanying, following, going as someone's companion, the placenta. (In Folklore) a spirit which follows someone; a guardian spirit.
In Norway one could very well argue that the most prominent scholars on the field are Leiv Heggstad (1879-1954), Finn Hødnebø (1919-2007) and Erik Simensen (1938), being professor in Nordic language science and associate professor by the institute for linguistic and Nordic studies. In their publication Norrøn ordbok (Norse dictionary) 5th edition we can read:
fylgja f. 1 følgje, det å følgje. 2 følgje, lag, Guðr II. 3 etterløysning (etterbyrd). 4 fylgje, vardyvle, vardøger, verjeånd. 5 stønad, hjelp, (gð) m. dat., sil. ak. 1 følgje, vere med; fylgjast, følgjast, gå i lag. 2 følgje, høyre til. 3 hjelpe, stø, halde på (e-u el. e-u at); f. e-s málum, tale eins sak; f. e-u fram, stø oppunder, halde noko oppe. 4 følgje etter; forfølgje. 5 følgje, rette seg etter. 6 leggje seg etter, leggje vinn på. 7 halde, drive på med (e-u el. at e-u). 8 trengje, drive på (e-u); f. at, klemme, leggje åt. 9 sitje inne med, ha. 10 leve i frillelivnad (-levnet) med, følgje (e-m el. e-rri).
We can very well conclude that our Norwegian linguists provide a far more complex and wholesome explanation of what a fylgja in fact is. To sum all of this up in English; It is a deity, but physical and well known as well. It chases, follows, is the twin, provides, gives, helps, supports and speaks your cause. It is the one you live "left with". It is the after-birth with the synonyms legkaka, barnsfylgja, fósturfylgja (leg is mother, kaka is cake, barn is child, fóstur is fetus). This deity, being a fylgja, the tree of life (Yggdrasíll (yggr (scary/ugly/terrible) drasíll (tree/horse)), the twin, the ancestor, contains the DNA (the blood heritage, physical and meta-physical) of the mother and the father, and thus all accumulated ancestors. This heritage is provided, transfered through the struggle of a nine solar months pregnancy, in three phases, and follows in the after-birth where it is re-incarnated in accumulation in the newborn. It is in fact physically killed/detached/sacrificed/replaced.
But, this guardian angel, being the ancestral blood memory (personified as the pagan light-elves) still follows, accumulated in you. The word fylgja even means in Norrøn ordbok: "your intuition", your "sixth sense", your "foresight", your "gut feeling". The voices of those accumulated in you, those in your blood that have lived lifes before you, and now are accumulated in you.
fylgjur are thus connected in our mythology to valkyrjur, sleeping maidens, totem animals (especially horses and wolves), ogresses (often in pairs (two, like twins - one protective and beautiful for life and one scary for death)), and nòrnir (the spinners of the threads of fate). They represent both callers for life and for death. The more known connected male deity is Óðinn and his wife is the more known connected female - Freya (in death: Hél).
Many so called "pagans" miss to understand that our Norse (thus Native European) heritage is animism. They get all confused and call us atheists, because we seem to mix in some bloody bodyparts in their messed up and one dimmentional (Abrahamic) view of what Native European and Norse heritage is. They miss all dimmentions of the realms, in fact all nine of them, because the allmighty creator, heaven and hell do not exist in Native European, Norse heritage and world view. They miss to understand that our heritage is ancestral cult, nature, balance, blood and honor.