Okay. Played a bit of the Terra storyline (about...15-20 minutes' worth so far? Finished the pirate quest), I've got quite a few things to report.
First of all, and perhaps the most pressingly, Fiona, mechanically speaking, is broken. And I don't mean in the "overpowered, breaks the game" sense, but that a good deal of her stuff does not work right, if at all. Her basic attack doesn't work at all, inflicting 0 damage after the one or two times in the scripted prologue battles, and I am suspecting this is in part why Whirlwind Blade is free, as a stopgap fix. This is worsened by the fact that these are the only two abilities she has that don't feed off of TP, which raises too slowly to make use of in most fights. Additionally, as-is, Feed the Sword doesn't work as advertised, if only because it doesn't drain her HP. But this is probably good, for reasons I'll go on a bit later in the post. She's also not helped by her fixed weapon, which really limits how well her damage can scale over the game.
(Also, on the player side, it is not fun to have the game advertise the character being able to use other gear if their current gear is locked. Especially if the other gear rates better.)
David has a similar issue. Locked gear everywhere (though his defenses seem to keep up) and not really much to do. The problem comes in where his focus (self-buffing in particular) doesn't...quite match up to the rest of his set. Fights aren't going to last long enough for those buffs to matter with his base damage values, even with their 10-turn durations. Also to be addressed in further detail down the line.
Will and Ana appear to work as intended, though Ana being the only one to get a weapon upgrade (that gives sizeable buffs to her magic, no less) basically means that by the pirate raid, she's the single most damaging character in the party. I'm not sure if Will's elemental variety so far is too meaningful? It feels like Jolt's good enough for the entirety of the spread of opponents here.
That is to say, I suppose, the balance between the Terras is...off. Though looking at things, in most cases it seems more as if the balance between physical and magical abilitites is off. Which...well, that will take a bit of a lecture.
Assuming you used the RMMV sample formulas here...well, that'd do it honestly. RMMV's sample formulas are based on the sort of approach that previous RM games do, with basing physicals entirely on stats while forcing magic to adhere to a half-stat-plus-static-value formula by comparison. It becomes egregious, though, when physical abilities (or basic physicals) break 100 on a good day, while magic easily smashes for 300+ without trying. On both sides.
The big problem with heavy static values on magic, especially using a subtractive defense is that the magic defense stat quickly becomes irrelevant outside of very niche cases (Fiona needing nearly 300 MDF to get the sort of resistance she gets says something. Especially when the enemies start spamming spells and whapping the rest of the party for 300-500 easy) and you get a weird case of HP as the primary defensive stat in most cases. This also leads to discrepancies in party effectiveness, where Ana easily does ten times as much as Fiona or David (before her second-rank spells), rendering them best off being item caddies in practice.
The matter with magic here is that it tends to make things quite...rocket taggy. The pirate boss just plain oneshots a party member if he casts Fire, no ifs, ands or buts. I suppose overall I'd suggest reining the magic numbers in, possibly going back over your math with a fine-toothed comb if you have to. (Or at least, revising magic attacks for enemies and physicals for PCs)
Other issues I've noticed include numerous points in the Terra path where it references or straight-up uses the Xiao characters' portraits. I suppose that's due to it having largely been unfinished, but it's worth pointing out, I feel. Mapping on the wartorn village also feels slightly off, and I'm going to echo Gate's issues with the feeling of pacing in the intro. We're dropped into our party at the start of the dungeon, no real intros or whatnot. At the least, it picks up a bit, but the intro dungeon leaves a bad first impression to me. The scripted encounters there, moreso, since they don't feel quite like they add much. Same with the scripted encounters in the pirate raid feeling like they're no different from the random encounters, just that they're visible. If you're going to have something visible like that, make sure it's a notable fight.
Last thing I'm going to point out, it may just be me but it feels a bit jarring that you're using the default item names (...even if we're down to four of them because MV pruned the living shit out of the sample material down to something ridiculously bare-bones)
[EDIT] Polish issue I forgot to mention. For some of the mook pirates in the hideout, the faceset you use doesn't quite sync up with the portrait grabber MV uses. And so when they talk they have like...four different faces shown. At the same time.
