It is unknown what this species truly looks like, as having the powers attributed to a God (or possible demi-God) they could feasibly change their appearance. They have been known to appear as a humanoid/Sebaceanoid species, very pale, in Roman-esque attire. However, when not coalesced, they seem to take the form of smoke or steam.
They are capable, however, of surviving in a specific region in deep space, again attributing to their God-like status. The Builders are an ancient and powerful race and are considered by Leviathans to be gods. In fact, the Builders originally constructed Leviathans as ordinary biomechanical ships, only later giving them sentience (souls) and the ability to reproduce on their own.
The Builders seem to be an arrogant species, but with good reason. Opposed to violence, they were disturbed to discover that Moya could produce gunship offspring. They ordered her decommissioned, but reconsidered when Zotoh Zhaan proved herself worthy of protecting and guiding Moya's responsibly.