You know, I would take them to be the gods of the earth, as it were, or what for the Greeks would be chthonic deities. I suppose in Japanese these would be regional gods, or anything of that nature - I don't know Japanese myth and legend, so it's hard to make a comment, but I would consider them like the minor deities of a specific location. Say, what in our myths are nymphs and centaurs and satyrs and the like. Creatures which are older than humanity, and are born of gods before the Olympians came to power, from the unions for, in the case of nymphs, water deities. In time, they become the 'gods' or guardians of certain locations, deities of springs and wells and forests. I should think that to be the best explanation of a Nu. If you look at them like that, as the Chrono version of nymphs, it makes quite a bit of sense, I think.
By the way, I just had a funny thought. You know that 'all life begins and ends with Nu. That is my belief... at least for now...'? It just struck me: that sounds like a Natural Philosopher talking, from some heyday of early philosophy where the philosophers were putting out this theory and that for what lies at the origin of the world.