I've always taken from the relevant passages that she was not human.
Specifically, that she was some form of water or river nymph, as both she described herself as the daughter of the River: '
Come dear folk!'
she said, taking Frodo by the hand. '
Laugh and be merry!
I am Goldberry,daughter of the River. '
as did Tom: By that pool
long ago I found
the River-daughter, fair young Goldberry sitting in the rushes.
(Both quotes from the Fellowship of the Ring .)
Letter 210 has a brief comment on Goldberry and her significance, which reinforces the fact that it's not likely that she is human: Goldberry represents the actual seasonal changes in such lands.
It seems to stretch belief that a human would represent seasonal changes when we do not see such conceptual representation by humans anywhere else.
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The wikipedia article states that her origins/race is uncertain but it does have this to say about her: Although Goldberry's origins are uncertain, Bombadil clearly identifies her as having been found by him in the river and her title "River-woman's daughter" strongly suggests that she is not a mortal human being , but rather a spirit of the river Withywindle in the Old Forest of Tolkien's Middle-earth.
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Frodo certainly reacts to her as something special: He stood as he had at times stood enchanted by fair elven-voices; but the spell that was now laid upon him was different: less keen and lofty was the delight, but deeper and nearer to mortal heart; marvellous and yet not strange.
I'm prepared to call her the river daughter and be happy with that.
There's no point trying to square Tom Bombadil with the rest of Middle-earth.
He's a delightful enigma, and so is his wife.
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According to Tolkien's Letter #210: We are not in 'fairy-land', but in real river-lands in autumn.
Goldberry represents the actual seasonal changes in such lands.
This fits with Tom calling her "River-daughter".
So she's a spirit of nature, as is Tom Bombadil himseld -
at least Tolkien said so in one of his earlier letters written before LotR. Later, he seems to have favoured keeping Tom's nature a mystery.
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Goldberry is a nature spirit, with one possible source given in the late essay "Of the Ents and the Eagles", which was taken up in the published Silmarillion, Chapter 2: "Of Aule and Yavanna": Behold!
When the Children awake, then the thought of Yavanna will awake also, and it will summon spirits from afar, and they will go among the kelvar and the olvar, and some will dwell therein...
(This is also a possible source for Bombadil).
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There are many quotations that suggest she is not a mortal human being, but in fact a nature spirit or a Maia.
From the Adventures of Tom Bombadil:"Go down!
Sleep again where the pools are shadyfar below willow-roots,
little water-lady!"She is clearly able to live IN the river, but also on land, suggesting that she is not completely human.
From The Lord of the Rings‘But
I see that you are an elf-friend; the light in your eyes and the ring in your voice tells it.’
Here Goldberry recognises the goodness in Frodo, but makes an allusion to the one Ring, hearing its power working on Frodo as she speaks to him as she has not yet seen it.
Her and Tom both seem to sense the ring and perhaps see the 'shadow' realm, meaning that they are more than simple humans living in an ANCIENT forest.
I think it unlikely that they simply humans.
Her singing, for example, is described "as young and as ancient as spring".
Personally I believe that her and Tom are nature spirits, sent to act as protectors for the diminishing natural world.
They live in 'The Old Forest' which many find an unpleasant place, which makes me believe that they have lived there since before corruption made the trees hateful, otherwise they would encounter many problems.
Furthermore, Goldberry's voice implies that she has lived close to the birth of the river, and has seen it through many seasons, giving her the appearance-and voice- of a youthful woman wise beyond her years, when she is in fact, immortal.
She also states that Tom Bombadil 'Is', meaning that he just exists, and is tied to the land.
As the world around us simply 'is' there, so is he, and he will only die when it 'is not'.
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Having read the Lord of the Rings a number of times I was always curious about the significance of Goldberry. I have not managed to find any reference to her origins within the other Middle-earth books that I have read.  Is she was just a mortal woman, or is there was some deeper significance to why she ends up living with Bombadil, or is she just a token female character designed to add a little balance to the overwhelmingly male character count?
Is Goldberry a regular mortal woman?
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 There are many quotations that suggest she is not a mortal human being, but in fact a nature spirit or a Maia.  This is inferred in the late essay "Of the Ents and the Eagles", which was taken up in the published Silmarillion, Chapter 2: "Of Aule and Yavanna":   Behold! She is more likely to be a nature spirit or a Maia.
She is more likely to be a nature spirit or a Maia.