The Silmarillion:Valaquenta

[Rían]

Originally posted by Dunadan
I seem to recall that not all of the Ainur chose to enter Arda. Is there any info in the secondary sources such as HOME about these? If they were involved in the Music, their personalities ought to be reflected in Arda.

You are correct; I replied to this a few days ago, but that was one of the posts that fell into the Internet Black Hole a few days ago, so here it is again:

from the Sil, Ainulindalë:
Thus it came to pass that of the Ainur some abode still with Ilúvatar beyond the confines of the World; but others, and among them many of the greatest and most fair, took the leave of Ilúvatar and descended into it. But this condition Ilúvatar made, or it is the necessity of their love, that their power should thenceforward be contained and bounded in the World, to be within it for ever, until it is complete, so that they are its life and it is theirs. And therefore they are named the Valar, the Powers of the World.

It reminds me of how in C.S. Lewis' (Tolkien's good friend) Space Trilogy the angelic beings are associated with different planets. Also, how the concepts of love and commitment go together; Ilúvatar didn't just say "well, just stick around Arda until you get tired of it, and then you can come back". They are bound to it forever; until the end.