Uru (ウル) is a family exclusive to Grandwalker Nexuses, which premiered in SD43. The family is dedicated to Battle Spirits originals, be it the characters created on the flavor text of sagas, or characters from the anime series.
Due to it not being as evident as the other families through their Grandwalkers, there's no source for what Uru is supposed to mean for sure, but a theory that was generally accepted was that it referenced Uruk, the first civilization to exist on the world.