FF10 HD: Middle of Thunder Plains. Was going to play more before analysis but...long overdue, so here we go!
First notable thing about Tidus is the Shoopuff stuff. First off, he awes at a city underwater, because hey, that's neat and unexpected! Wakka quizes him on the "Why" and Tidus gives a simple logical answer of "Well, it's really convenient with all that water" and Wakka basically goes "NO! YOU'RE WRONG! THEY WANTED TO PROVE SOMETHING!" Now, here's something I didn't remember; Tidus didn't just take it and move on, he actually went "I'm not so sure..."; he's basically avoiding an argument, but at the same time, expressing he disagrees. This exchange, small as it is, shows two things about Tidus' character:
A. He's got half a brain. Tidus isn't what I'd call a "smart guy" but he's also not an idiot. "Average Intelligence" sums up Tidus pretty well, so while not winning any awards on intelligence, he's not going to trip over his own feet and can put things together. It's rare we see this since so many games either aim for displaying the characters as bright, or just downright morons; Tidus actually is believably middle ground.
B. Tidus isn't simply just eating all the crap he's being spoonfed. Wakka, very confidently, tells Tidus that he is wrong, and Wakka as we know is brainwashed by Yevon like the rest of Spira, Al Bhed not withstanding. Nonetheless, Wakka shows every bit of confidence in his argument, while Tidus is more just proposing ignorant speculation because he doesn't actually know, just "Well, I think this." Thing is, rather than going "oh, I see, guess I was wrong", Tidus' "I'm not so sure" response indicates that while he doesn't know the truth, he isn't convinced that Wakka, as confident as his claim is, is right.
After this, Tidus is pretty quick to point out "if Machina is evil, why do you guys use it for things like Blitzball?" which is fair, but thankfully Lulu responds this time with a logical "it depends on the Machina, Yevon tells us." This is less a Tidus thing and more the game covering its own setting with hypocrisy, using Tidus as a logical point to question it, which is exactly why Tidus I think works as a character, as on top of being a character, he's also a proxy for the player; he knows as little about Spira as the player does, thus asks the questions he asks.
Fight with Rikku, then, another quick exchange happens, with Wakka being all "AL BHED ARE THE DEVIL!" rants because racist. Tidus, much to my surprise, said something I don't remember him saying, but nonetheless, great line to use for people who claim Tidus does nothing but whine; he tells Wakka to stop the useless complaining and just focus on protecting Yuna. Yes, Tidus, the alleged whiner, just told Wakka to stop his whining and focus on the task at hand.
Nothing much happens outside of Guadosalam, where it's mostly Tidus speaking his mind. This is Tidus being naive as he's still not use to the fact that Maester = Super Important Role. He doesn't like Seymour, so he says disrespectful things in his own house (much to Yuna's dismay.) After that, we get the Farplane.
Tidus whines about Jecht again, the one thing he actually does whine about in the game, and to this point, the game tries to establish that Tidus' complaints are justified because Jecht was a total douche. Yuna questions why Tidus hates him so much and Tidus first responds "Everything he does pisses me off!" but then in a surprise, comes to an even more concrete conclusion, that being Tidus loved his mother a lot, but everytime Jecht was home, she'd completely ignore him in favor of his father. IOWs, he has a legitimate Oedipus Complex, wanting to spend time with his mother but competing with his father. In discovering this, he comes to a realization about Jecht as well, that Jecht wasn't entirely a bad person, as he remembers his father sometimes going "Oh, spend time with him, he'll just cry about it if you don't."
This scene is important for two reasons, one being Tidus and the other being Jecht. The Tidus reason is that Tidus finally starts to recognize why his father is an outlet of his anger, and it's the first time he ever looks at his father in something more than just pure negativity. It's not necessarily positive, but it's the first time Tidus started having shades of grey, which in itself shows Tidus is just now starting to doubt his own beliefs. The other side is Jecht; until this point, all the Jecht scenes we've seen were purely from Tidus' perspective so "This guy is a total dick!" Yet somehow, Yuna is capable of finding him "kind and gentle" and it's implied Auron respects Jecht as a friend, so there's obviously a disconnect. This is the first instance we see that maybe there's more to Jecht than meets the eye, which we will later discover Jecht wasn't so much a dick as much as just bad at expressing himself towards his son. In this scene, we see Jecht actually telling his wife to spend time with their son instead of himself, a very fatherly thing to do, but with the "he'll cry about it if you don't" demonstrates that he's just bad at giving a good reason. Obviously what Jecht meant was "he needs you more than me, spend time with him" but said it in a poor, demeaning way. Basically, heart was in the right place, but his ego got in the way. This is the first hint towards the fact that Jecht is a legitimately good guy, just is too full of himself at times to express it properly.
The rest of the sequence is mostly Tidus probing Lulu for info about Yuna, and him thinking marrying Seymour is silly. It doesn't really show much of Tidus' character other than I guess being an early precursor that "yes, the two have a budding relationship." I mean, why would Tidus care so much about Yuna getting married if it's just for PR? I guess you could argue "because Tidus wants his friends to be happy" but yeah, this sequence is mostly Tidus being a proxy of the player again; marriage is not a simple thing to commit too, most players are probably thinking "DON'T DO IT! YOU BARELY KNOW THE GUY!"
Not much in Thunder Plains, outside of Tidus probing Auron for "So...that's all you have to say?" and Auron's response of "The pilgrimage will still go on, I have no objections." Fair, Auron just wants to get Yuna to Zanarkand (the reason of course is not obvious on a first playthrough, but that's not important here), so makes sense he doesn't care what Yuna's decision is if the pilgrimage keeps going. Tidus' reaction is less whiny, and more "seriously, you're really going to go through with it? Just like that? IT'S MARRIAGE!"
...yeah, I had a lot to say for such a small part of the game apparently, guess it's a good thing I talked about this now!