I was gonna say Dawn and May and Ash of course but I think they've all been covered in this thread a lot already lol.
I totally agree with the Team Rocket trio development, even if (or especially because!) it's subtle when you're not paying attention. One thing I noticed is that in "The Problem with Paras" really early on, when Meowth gets sick, Jessie and James almost shrug it off and leave him behind - but in "Operation: Dub Pikachu" recently, they both immediately try to help him get better. Jessie also started caring for Wobbuffet a lot more over time. They all seem like they stick together and stand up for each other a lot more (and aren't quite as averse to helping out the twerp sometimes as they used to be)!
I also personally really like how Gou's character is developing right now as well. Obviously Journeys is nowhere near ending, so maybe they'll throw a curveball at us (pun intended) somewhere in there and I'll change my mind, but I think it's being handled pretty realistically. He still struggles at times with a lot of things because of bad past experiences, but is slowly trying to learn and get better, instead of just making a complete instant 180 like I sort of expected at first. Even Koharu pointed out that he's only just now starting to rely less on data and calculated outcomes, and more on instinct instead. That kind of growth is really investing to me.
I think I've typed enough but obviously, honorable mention to Gary too, even if everything that could be said about him has been said already. I always think that "The Rivalry Revival" is a really good example of that off-screen development, since that was the first time we saw him after his loss at the League, and it's noticeable that he's changed since then.