If we count Pokémon too, Lana's Eevee comes to mind.
I'm convinced Shaggeon left at the end of its episode, leaving behind a Substitute Doll and nobody's noticed the swap
As for characters without character development...
For Alola, it's definitely Mallow, who keeps saying she wants to make Aina Café popular, then does nothing to actually promote it
I'm sure at the end of the series she'll magically get a lot of customers because she participated in the League though
Instead of doing something actually related to cooking, like becoming an actual rival to Team Rocket in selling food
Lillie's up there as well, since after being able to touch pokémon, she didn't do much of anything until she FINALLY got a Z-Ring and took the time to master it, resulting in one of the single best animated Z-Moves in the series because of course she gets the most effort dedicated to making her look good while sacrificing others to do so
Also took Kiawe getting a second Z-Crystal to (vaguely) help him claw his way out of the flanderization pit he fell into during the first year where he goes from cultured Alolan to screaming hothead
And then Sophocles literally does nothing for two and a half years before finally evolving Charjabug and getting tortured by Ash and Kiawe while learning how to use a Z-Move
(Contrast to the girls, who are in the lap of luxury while they train)
Say what you like about previous regions, at least they kept characterization consistent