"Out of the discords of the Music - sc. not directly out of either of the themes, Eru's or Melkor's, but of their dissonance with regard one to another - evil things appeared in Arda, which did not descend from any direct plan or vision of Melkor: they were not 'his children'; and therefore, since all evil hates, hated him too." (Notes on motives in the Silmarillion (iii), Text VII Morgoth's Ring)
From what can be gathered from the Silmarillon, she was an Ainu but not allied to the Valar, nor to Melkor. Period.
In sil it was said something about ea being a small kingdom. Perhaps ungoliant came from some other realm.
Virumor — but who knows how CJRT edited that portion of the Silm.? HoME shows the Silmarillion in its various stages — indeed, that quote itself was from a note commenting on a portion of the Quenta Silmarillion found in MR. I don't understand how providing more than one theory for something is less than ideal — the people of PT may have come to a consensus that she was a Maia, but not the entire online Tolkien community agrees.
Since her power includes things related destruction like Mogoth's...she could be one of his people in the timeless halls before the music was even made...she definitely was not very powerful at first as she did fear Morgoth when he first came to make a deal with her to help him smite Laurelin and Telperion...but after draining the tree's light and the dwells of Varda,she became more powerful that even Morgoth neede the aid of his orcs to prevent Ungoliant from taking away the silmarils...