Let me just preface this with spoilers:
I’m replaying, and while I haven’t finished yet, I’m at Capua Nor, post Heracles cannon. And even if Yuri doesn’t really develop himself, the game takes great glee in his double standards, Pharaoh was practically the conversation between him and Flynn about Cumore:
Pharaoh: “Sometimes you must cut off an infected part to save the whole.”
Yuri: “What gives you the right to judge, to determine when a part is infected and needs to be cut off?”
Nevermind that despite Yuri explaining the same sentiment to Flynn, Yuri said, almost verbatim, the exact same thing after the Don died about Judith (er, I think, I may have gotten the context slightly mixed up). I think he also has a little more development than you give him credit for, such as Yuri “punishing” Schwann/Raven in the Heracles by making him join up with them, whereas old Yuri probably would’ve cut his throat for selling out the person he’s almost overzealously protective of. It’s not much, and it’s subtle, but I think between many points in the game he alters his viewpoint at least every so slightly.
I have to day though, while I don’t hate Karol as much as you do, I do find him sort of useless, not just combat wise, but PLOT wise. Like they wanted to make a foil for Yuri but forgot Flynn existed, or something. He’s like what would happen if you took Krillan from the Team 4-Star Dragonball Z Abridged and actually gave him responsibility. But hell, if this were Final Fantasy XV, Karol would be the main character given how often it seems like they put the secondary character in the main role (was there a reason for Vaan to exist? He mostly just seemed to be there. I mean, yeah, his brother died in the opening and blah blah, but really. That seemed more like an excuse to make him the main character than a real reason for him to be plot relevant. At least Tidus was related to Sin, but Yuna was still a much more logical protagonist).
I didn’t mean to imply it was on the same order of magnitude… but it’s still pretty bad. For a game that’s so well designed in most areas it can have such weird flaws with the sidequests. Take the Dog Map sidequest, unless you use a checklist you’ll never figure out where you have left to mark. For Raven’s Ladies Man title, unless you’re anal about talking to every random NPC after every minor plot point (which you would in most games, but as both a blessing and a curse in Tales games, there’s usually too many for it to be feasible), it’s unlikely you’ll get that. There are some time sensitive quests that are like “If you rest at this inn there’s a 33% chance a scene will trigger, do this 6 times” (Karol’s Special skill), or “rest at an inn to get this but if you want to get both scenes, you must leave town and come back in between rests” (Dark Enforcer/Portrait of Nan).
In GENERAL if you get a sidequest, it’s pretty good about alerting you where to go next, even if it’s not clear WHEN you should go (Ehmead Hill grave, you know to return, but not when. Guild Quest pt 1, after you get it, you can’t complete it until Judith comes back though it never mentions you need your whole party), but some quests can get pretty bad with their arbitrariness, especially the ones that are like “go back to this minor area you went to 20 hours ago and take a fork with a chest you probably already opened way back then to trigger a scene to get a flower.” There’s encouraging backtracking (new areas you can get to with an upgraded Sorceror’s Ring) and then there’s arbitrariness. It’s pretty good usually, but once the game opens up after Fiertia/Ba’ul, sometimes they put quest triggers in the oddest places. Luckily the Item Book bullshit doesn’t really contribute to the story or that’d be a whole 'nother rant (mostly revolving around the Karolean Sword +1).
I can excuse the bullshit secret missions, they’re meant to be hard and annoying, but putting quest triggers in a place you never have a reason to revisit is a bit… odd. That’s not to say I don’t like the game, I love Vesperia, and nothing in a JRPG will EVER be as bad as 100%ing X-2 or getting the Zodiac Spear, it’s just a bit silly sometimes.