The overarching plot wasn't handled super well. It was like it started as a character-centred game and then, suddenly, at the end they're like "actually we want you to save the world" - the stuff always going on in the background and your inability to involve yourself in it was not at all conducive to the finally they had written. Before that though, when it was mostly just about the characters undergoing their various journeys, it was nice.
Character-wise, Hop is what Hau was supposed to be. He's a nice guy, but he's also been living in Leon's shadow for so long that he defines himself entirely by his brother's success and constantly compares himself to him. Somewhat less explored, but he's also constantly occupying this unfortunate juxtaposition of trying to be a supportive friend and trying to beat you. His overall character arc, including the post game where it actually ends, is very satisfying. He goes from defining himself by his brothers success, to his defining himself by his inability to measure up to that success and then, finally, to accepting his individuality and choosing his own path.
Bede is amusingly similar to Hop, but the jerkass version. He defines himself entirely by his selection by Chairman Rose, but instead of a buried inferiority complex - because Rose isn't a trainer really - it gives him a superiority complex instead. He thinks because he was endorsed by Rose, he's destined for greatness and he bases his entire identity around that. He's also supremely grateful to Rose for what he's given him, so he goes to extreme lengths to prove his worth. As the story goes on, he gets abandoned by the man he thought he idolised and thrown out of the league, he has to find a new path and an identity for himself.
Marnie is kind of the inverse of the other two rivals. She's already carved out her own identity for herself in a way, a much healthier one really. She's got her own personality and her own mission. The problematic stuff comes from Team Yell, but I'll get into that later. Again though, thing don't go the way she wants and she has to adapt, instead following in the footsteps of Piers. Marnie did have a problem though: lack of screen time. It was difficult to explore her character since she didn't really appear that often comparatively and a lot of her character was through the lens of Yell's idolisation of her.
Speaking of Yell, they were more characters who defined themselves entirely by their relationship to another character. To the point of an unhealthy obsession that lead to them doing things that made life difficult for everyone around them, even though they had good intentions. Oleana is similar, she's totally dedicated to Rose, just like Bede, and never questions him - to the point of idolisation really. She's a villain simply because of her dedication to Rose and mission to make his plans succeed. Her entire life revolves around him. Rose himself has an obsession of his own, but instead of a character it's a problem. The energy crisis he talks about is very real, but he becomes so focused on the problem of the future that he's willing to sacrifice the present.
Those are some pretty deep and complex characters, especially for a Pokemon game and one could easily write compelling stories just following any one of them. Perhaps the biggest limit on them is that you only get to view them through the eyes of the protagonist. Plus, there's interesting recurring themes throughout the game, because of these characters, - individuality, finding your own path and the dangers of obsession and fanaticism.
I haven't even touched on Sonia who probably has the best arc of all of them, but there you go. The actual plot wasn't particularly amazing, but the character writing was really solid.