Some explorations you might enjoy: http://lotr.wikia.com/wiki/Theories_about_Tom_Bombadil
As a writer, perhaps Tolkien didn't even specifically define the character, other than the spirit of vanishing nature. Since he indicated he meant for it to be an enigma, perhaps spirit of disappearing mystery might be more appropriate? ;-) Sorry for being a bit trite, but such is the nature of the character!
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