I use grays in my TU. they exist primarily as an elusive, secretive species, nobody knows quite where they're from or where their homeworld is, and encounters (At least, with evidence) are extraordinarily rare, usually occuring on backwater or frontier, lowtech worlds.
Grays: Amphibious humanoid aliens
Ability score adjustments:
STR -3
DEX +1
CON -2
INT +2
WIS +0
CHA +0
EDU +2
The race is ancient, and technology has advanced so far that the race has evolved to be physically weak and fragile, however their intelligence and memory is incredible. Rumors of psionic abiliites have never been confirmed, but that doesn't mean much when dealing with a race nobody knows anything about for sure. They reportedly stand between .75 and 1.25 meters tall, and have a mass between 15 and 30 kg.
Civilization TL: 17
Organization: Usually encountered in small crews of 2 or 3. Technology tends to be highly streamlined and ergonomic; ships are saucer shaped, as are many vehicles. Teardrop shaped pods are also not uncommon.
Nobody is entirely sure what their purposes in entering inhabited space are; they never stay long and never make concrete contact, avoiding military and scientific facilities. They have only rarely shown any overt aggresion, seemingly interested more in research and observation.