Yeah the cast this time around was pretty good just like the story, which is a nice breath of fresh air after the horrific XY one.
Saying what haven't already been said about Lillie, Nanu and Lusamine is a daunting task at this point. I liked Lusamine a lot, liked to see her as the true villain of the game and this line: "Terrible? Me? How am I different from any Pokémon Trainer, like your little “friend” there? What do you do with a Pokémon you can’t use? You remove it from your party, as you please" is one of the best ones in the entire series. The only thing I didn't like about her, is the fact that the game doesn't make it really clear why does she want the UBs so bad.
Liked to see that the Kahunas and Trial Captains were more than just some random elemental trainers that just stand in one place waiting for the players to whip their teams just like most gym leaders, they actually did a few relevant things in the story and also had their own goals and lifes, they actually reminded me of the BW gym leaders which were character-wise the best selection of gym leaders in my opinion. A nice example would be Kiawe, the fire type captain, which you discover by doing some kind of sidequest that he worked on a market in order to get money to study fire dance abroad, it actually made made me feel connected to the character, something I don't usually feel on a Pokémon game. It felt good to see these guys as actual humans and not just random NPCs...shame there was one character without such luck.
I felt dissapointed about Kahili, the flying type E4 trainer (which I wouldn't blame you if you have forgot about her), she is the only character in the E4 which you don't meet before this challenge, the closest you get of her before this are some people talking about her in Hano Grand Resort being a very powerful trainer and golfer, she seemed to have potential as a character because she also completed the island challenge just like you and had a rather interesting more serious personality from what I have seen, but she ended up falling in the "random elemental trainer" archetype...the boss battle against her was pretty cool tho. EDIT: the only good thing about her is that you don't fight her before the E4 making her not feel like a weak trainer but I don't know if that would make her a better character or a better boss.
Overall the cast was pretty good and I hope for Arceus that Game Freak makes the same with the gen 8 characters and don't go backwards like they did with gen 6.