Usually I claim Tom as being a nature spirit, either the spirit of Middle-earth or as a representative of the Spirit of England - a bit of a Genus Loci, with a touch of the Gog and Magog going on.
But right now I'm having some fun thinking of Tom as a Timelord. Never aging, resistant to the Ring, a definite eccentric, he seems to exist outside the rules - so why shouldn't he be a Timelord? Which version of the good Doctor he is though, I wouldn't like to say, though he is quite Tom Baker like.
Goldberry of course is the ultimate Assistant.