Well, Lillie is the obvious choice here. Her arc is so sweet and she's gotta be the most developed Pokémon character, other than maybe N.
Lillie really surprised me on how much I liked her character and arc. Skipping to the wild theories when she was first revealed, when playing through the game I liked how, despite not being a trainer (something she annoying likes to keep pointing out) she did all she could to help Nebby, thus showing to be a proactive person. This came to a head after the whole Aether Paradise incident where she sheds her shy personality for her "Z-powered form" one (a name which would have made more sense had it been "Mega form" as Mega Evolution is the transformation up-grade technique not Z-Moves wich are just a super attack, but I digress). She goes through Poni Valley by herself, awakens Nebby's true potential with help from the player, and finally gave a talking down to her insane mother (and eventually freed her from Nihilego control).
Her relationship with Nebby and to the player was sweet; her care for Nebby showing she was not only kind-hearted and determined but also meant to be compared and contrasted with Lusamine's: Lillie is in practice what essentially Lusamine says she herself was, a person who wanted to protect Pokemon with their love. Her relation the player helped give the player a place in a story as not only muscle but also as an inspiration to Lillie to take that "Z-powered form" step and confront her mother. And her side relations such as with Professor Kukui, Burnet, and Hau were also sweet. I only wished they may have found a way to expand her relation with Gladion, we only get a short bit about Lillie being angry she left her when he ran and that's really it. Maybe at some point have it hinted Gladion sent the third Type: Null to Lillie, trusting that when she becomes a strong trainer she'd be able to use and evolve it much like him and the player.
Overall Lillie is a great character, her story arc being right up there with Silver's, N's, and Emma's.
The man dressed as Machamp in the Hano Grand Hotel.
The man dressed as a Hypno and asked money from you.
Great Japanese humor.
I'm sure we can list off a batch of the one off NPCs. So how about the one group that really stood out to me: The Eeveelution Users.
Sun & Moon brought a lot of hard facts of life into the Pokemon world, one being about the effect growing up has on you. We always saw these games through the point of view of a kid/teenager who's ready to take on the Pokemon World, but what happens after that? Well we see most trainers who can't be masters just live normal lives and making it by as we all do. Kagetora was a strong Eevee user but was never able to get beyond that point and eventually settled down, started a family, and is working a job to pay mortgage bills. But it gets sad (or rather sadder) as we face the other Eeveelution Users who are all elderly. One has Alzheimer's, another is constantly in and out of the hospital (though this has been a life long problem for him), one used all her money for medicine and operations to keep looking young but she still had old week bones, and one even passed away so you had to fight her granddaughter instead. Its a really grounding side quest, the point being you should make all you can with your life as its fleeting.
That woman being held by Machamp and her story about her dead husband. That was unexpected from Pokémon and I loved it. More stuff like that Gamefreak.
What 0_0
I missed this person, could you sum up their story?
Oh, but it gets worse. For you meet this old woman later in the game: she's Hapu's grandmother. Meaning the man who died was Hapu's grandfather, the former Kahuna of Poni Island.
Nanu. Between having an actual aura of couldn't-care-less and ditching his own Elite Four role because he felt like it, he's definitely one of the most memorable characters, if not the best just because of that personality.
Also
While I like Nanu for his non-caring attitude, for me he really came into his own in the Looker Missions. He has a lot of hidden depths, and though he acts like he doesn't care he actually deep down does, which is why he stays near Po City and helps Looker to assure Abigail's safety (and prepares the player via a battle). He also has his goofy side as we see during the times he does his Z-Move pose.
Another interesting thing about Nanu: he and Molayne (Sophocle's cousin) convinced her to take the Pokemon League challenge. While its probably them trying to encourage a strong trainer to get out of the rut she put herself in, at the same time I wonder if at least Nanu is trying to find someone else to be Kahuna. He's only Kahuna because you can't really say no to a Legendary choosing you, and had Molayne been worthy he would have been chosen already. So maybe helping to get Plumeria on track would get Tapu Bulu considering her (at least in my headcanon, I also think Guzma should become Champion after these games, not because he wanted the position but his reason for taking it is that he's "holding it" for someone he wants to battle when they come back (thus hinting the player character, like all the others, had left on another journey)).
Maybe I'll comment on other characters later and add my own. I'm surprised no one had mentioned Kukui, Hau, or Guzma yet.