I'm pretty sure Kartana is Mohn.
Celesteela might be Faba because of the color scheme and technology theme, but I think it's more likely Kahili since they have the similar color scheme, feminine appearance, carry rods (golf club and bamboo blasters), and matching flying type (though that means nothing since it's not like Guzzlord or Xurkitree match types with their human counterparts).
And if Necrozma has a human counterpart I think it would be an inverted Kukui, Kukui with his white lab coat and Necrozma being black. They also have rainbows as their spot color (Kukui on cap, Necrozma on face) and they're both kinda like the "last boss". Wild guess on this one though, I'm totally not sure. It's not technically an Ultra Beast, but it's got an obvious connection to them.