Skipped:
Youngster & Lass: I skipped them initially because we already seen their art, but I guess they're still worth a mention.
Youngster looks mostly the same, just more detailed He has a Sableye on his yellow shirt now and his pants are a darker blue with a white stripe.
Lass on the other hand looks completely different. I'm telling you, they have a whole draw of unused female characters...
XY Reused Art:
Swimmer (Male): As I mentioned, the male Swimmer is the same one from XY, but that's mainly because the male Swimmer in general hasn't changed much ever since his introduction in GEN I. Okay, there is some differences, like he's now wearing a black cap with a blue Pokeball logo, his goggles have white lenses, and he's wearing black shorts instead of a blue speedo (thank Arceus); but he's still a shirtless, muscular guy who who find swimming the world's oceans (some are so good at it they can swim region to region!).
Hiker: WHY IS HE WEARING A FEATHERED BOA? Actually, oddly enough the XY Hiker looks remarkably similar to the original Gen III Hiker more than any other Hiker. The only difference is his jacket is open, his pants are a darker brown, backpack is blue (and we can see his sleeping bag), has a neckbeard, a leaf in his hat instead of a feather, and of course an out of nowhere feathered boa. But every else matches, the green jacket, green socks, the feathered boa is the same color as the Gen III Hiker's ascot even. Maybe this was done on purpose?
Black Belt: The dark hair Black Belt actually began being used in Gen III but switched over to a brown haired one in Gen V until switched back in Gen VI. HGSS also had a slight adjustment with their Black Belt with longer hair and no red bandana to match the original Gen II sprite. The XY Black Belt has slicker hair and sideburns compared to the Gen III Black Belt, but they're overall the same character being the generation which debuted the dark hair Black Belt.
Hex Maniac: Another one with two sprites, though there's an obvious reason for the international change. The Japanese Gen III Hex Maniac has no pupils, kind of a bit freaky so international made her looking down into her crystal ball. Speaking of which, obviously they don't really match, which is kind of funny since the Hex Maniac debuted in Gen III. To be fair they sort of look more like Gen I's Channeler's due to the wild hair and eyes.
Psychic: Another odd case of missing females, XY only brought a male Psychic while previous generations also had a female one. Oh, and YET another one with different sprites, except this time for the girls. For some reason, the Japanese female Gen III female Psychic sprite hands are inwards while the international was changed to have them outwards. Okay? But yeah, they don't look the same since he's probably based off Gen V's Psychic, though him and the Gen III female Psychic have a similar hair style.
Twins: though this generation was the one to introduce Double Battles, they decided to not change the Twin's artwork to match Gen III's. FUN FACT: The Twin trainer class was introduced in Gen II, however they were a normal battle. When Gen III introduced Double Battles they only ever did Double Battles from then on.
New:
Divers: First up are these guys which we'll call Divers. They're not actually a surprise since it was announced you would be battling trainers underwater. I'm also going to guess that even though you're in the water you can still use any Pokemon you want. :P Anyway, they look alright, though are we suppose to take is that the girl is holding her breath while the battling is going on? In the trailer they showed that even your character puts on goggles and a breathing apparatus!
Plusle & Minun Bosses: No idea who there two are, but they look somewhat important. Maybe these two have to do with the Battle Resort, maybe the big bosses there? Eitherway, some of their toughness does melt away since they have Plusle and Minun on their back, but I'm guessing that's probably because they compete with Tag Battles with one another. We'll just have to see when the game is released.
Last Notes:
It's kind of odd they only have 2 of the same "meek" versions of Wally even though we know he has a "stronger" version of his art which he probably switches to later in ORAS. They also have two sets of the Ace Trainers for no particular reason.