UB-01 is not Lillie but it's so sad to see people think that.
First, the description of Ultra Beasts and UB-01 tell us it can't possibly be or even act like a human. "While evidence of something like a survival instinct can be observed in UB-01, no one knows whether it has a will of its own or any emotions" On the contrary, Lillie is a human with her own will and can clearly feel emotions. "she's working as Professor Kukui’s assistant for personal reasons. She's not fond of making Pokémon fight in battles, but she loves reading and has devoured many books."
Also, "UB-01’s body is composed of a glass-like substance. However, it’s constantly changing shapes, never settling on one." We can see that it mimics Lillie's appearance, but this shows that it could have never settled on a form that completely resembled her.
Second, there's common sense. If Lillie was meant to be this mysterious creature called UB-01 in disguise, the developers wouldn't make it look that obvious in the trailer. The developers aren't stupid and we're not stupid. There's obviously a reason why UB-01 acts like Lillie, but it's clear that they aren't the same being.
So here's my theory. I say Lillie is related to the Aether foundation's Lusamine and was part of an Aether experiment that had to do with Ultra Beasts. The result of that experiment, UB-01, is now connected to Lillie some way. Whether it was intentional or unintentional, UB-01 has clearly seen Lillie at least once. Amorphous beings like UB-01 tend to take shape of what they see and like, kinda like the common ditto, or Latias from Pokemon Heroes. Maybe Lillie and UB-01's relationship is like that of Tory and the Deoxys's energy form from in Destiny Deoxys. It's all speculation at this point, so anything's possible, except that UB-01 is Lillie.
many Pokémon in the anime were able to take human appearence (Latias in the movie) so maybe UB-01 remembers Lillie and took her appearence... who knows? Or maybe Game Freak wants to mislead us 
