Remember, he's not a
mad doctor, he's just angry. Comments from X-COM Command have been appended for a more in-depth view.
The Ethereals challenge everything we know about conventional physics and biology. We have determined, primarily through interrogating other aliens, that the Ethereals are the leadership of the invasion. Our one Ethereal capture proved more trouble than it was worth, but we learned a lot about psionics during its stay in alien containment. Mainly because it kept seizing control of or panicking HLPX-COM personnel. Interrogating it was impossible; it would not cooperate and had no spoken language.
The Sectoids are the Greys of myth. We have no idea how long the Alien War has been going on from their perspective; it could be centuries. If so, they rank among the worst military minds in the universe. Sectoids are short, they average perhaps a hundred and thirty centimeters. They're also definitely cloned; we've recovered well over three dozen bodies or live Sectoids, and only encountered four unique genetic sequences.
The Sectoid Hierarchy is subservient to the Ethereals, but in what fashion we don't entirely understand. They have no spoken language, communication is all psionic or by computer; we interrogated them over what amounts to instant messaging. The Sectoids were largely uncooperative, but we did glean something of their culture and daily lives. Short answer: they don't have any. Sectoid society has given up nearly everything in pursuit of efficiency.
Despite their bizarre biology, the Snakemen are actually the most human of our opponents. They were made subservient to the Ethereals about seventy-five years ago, by force. They have a culture which, while it glorifies the hunt, is not so dissimilar from our own as to be unrecognizable. Our captive Snakeman has actually learned to speak English and has generally been very cooperative and friendly; most everyone on the staff at Area 51 has been in to talk to it at some point.
The reasons behind this are as disturbing as they are poignant. Our Snakeman captive was a Rookie serving in the Snakemen version of HLPX-COM when the Ethereals conquered their planet. It wants to see us succeed where its own organization failed.
The most obvious thing to note about the Floaters is the drastic surgical and cybernetic alterations made to them, including everything from the waist down. As far as we can determine, these alterations are a part of their military service and are performed on enlistment. An unaltered Floater would be huge, nearly three meters tall. Their alterations are as much practical in allowing them to use standardized equipment as they are tactical in allowing them to fly.
Floaters are intelligent, but admit they are uplifted race of nonsentient animals. Where, when, and how is not clear, but considering their service to the Ethereals the "who" is easy to guess. Their culture is stunted, and performs support roles for their military; even their art is focused entirely on this purpose.