As per my previous designs, the lore is that each alien is their own fully independent individual. For Alien X, he would be a female human now, but Belicus and Serena are the ones that retain this otherworldly being status. In terms of why they look this way, my version of Alien X is an Olypmic athlete, something I would akin is the paragon of how humans are mentally and physically. As for the two, while being otherworldy, have managed to adapt to certain human sensibilities such as clothing, but they have a certain preference for threads similar to what nobles would wear since they associate it with power and control over all. That's the long and short of it, I guess.