To get back on-topic from all the "nature of Bombadil" stuff . . .
My good twin? Well, I always envision him with a sharp object or three stuck in him (downright
nauseatingly cheery, that one is).
More seriously, I think of him as . . . well, a semi-paternal figure. Wise and clearly old, but so active and cheerful that you don't really notice that. The sort of person who always knows what to do and has seen it all, and knows better than to be too serious. This is a bit more of a character description than a physical one of course, but that's how I see people - I probably couldn't describe my own parents to you in any detail, but I could tell you all about them.
