I've said this elsewhere, but it's most relevant here.
My theory is that, as others have suggested, Aether is more sinister than they appear. Them saving Pokemon from Team Skull and then whisking them away to the ultra high-tech paradise all seems a bit too good to be true. I am expecting some kind of Cipher-Snagem relationship, where Aether has the good public face and big fancy facilities (a la Cipher with Es Cade and the Realgam Tower), where as Skull is their thuggish frontline who attempt to steal Pokemon so that Aether can "rescue" them and take them away for their experiments, echoing Snagem's supply chain of stolen Pokemon for Shadow experiments. Perhaps Aether even created Type: Null and loaned it to Gladion the same way Cipher paid Snagem in Shadow Pokemon.
Type: Null was designed to fight legendaries, but we don't know which ones. Its crest reminds me a little of Tapu Koko, and the mask kind of echoes his big face shell. I hypothesise that Guzma may have commissioned it as the ultimate answer to all the different types of the Island Guardians so that he could defeat them once and finally get revenge for being kicked out of the trials.
Why Gladion is in control of it when we meet him, I don't know, but he may be a close ally or relation of Guzma's. Though I am growing more convinced that he, Lillie, and Lusamine may be related.