I had tons of fun with the battle system, it was just engaging and satisfying to stagger enemies so if it wasn't drastically changed in XIII-2 or LR, then I guess I'll play them.
Then I'd keep at them. If there's one thing I think the XIII series does well is the battle system(s). XIII-2 is a refined version of XIII's battle system with minor tweaks (mostly for the better). The only bad thing about XIII-2's system is the game is fairly easy compared to XIII because you can get overleveled now. In XIII, you were restricted by the crystarium (which was locked until certain points in the story), but in XIII-2, you're free to fight as much as you want and level as much as you want. There are some optional harder encounters later in the game (in particular the final boss can be quite hard depending on your levels/paradigms), but for the most part, it's easy.
Lightning Returns has a very different battle system from XIII and XIII-2, but it's still really fun. Quite challenging, too (especially if you play on Normal).
As far as the story goes, there are aspects to the story in each game that I found interesting, but the execution was very flawed. XIII-2's story is mainly entertaining to me because of the Noel/Caius/Yeul dynamic (I know you have no idea who those characters are yet though lol).
Final Fantasy XIII is... a weird beast for me when it comes to the characters. I first played it back when I was an angsty 15 year old who was able to relate to the angsty Hope and pissed off Lightning. I also liked Sazh. I hated Snow, couldn't stand Serah (about the only thing that hasn't changed... though I dislike her for different reasons now), didn't like Vanille, was okay with Fang. However, now that I'm older, my perspective has changed. I find Hope to be a raging, homicidal dumbass who I can't stand; find Snow to be a good guy (bit of a dumbass, cliched, and a little annoying... but the guy really just wants what is best for everyone. I'm kinda like "You're a stand up guy, you big lug... just cut the hero bull and stop with the Serah stuff"); have come around to liking Fang (she just has no fucks to give for most of the story and its refreshing when everyone else is on the brink all the time. She might not be the most consistent, but at least she is a break from the melodrama); like Sazh pretty well still ; and am fine with Vanille. Lightning is... eh? I mean, I still like her in XIII but I can now see why a lot of people wouldn't. Despite what the writers say now, what I always took from XIII was that Lightning's character arc was for her to realize that she wasn't "super mature soldier extraordinare" like she really thought she was, which is why I can accept that Lightning really wasn't a good person for about 40% of the game. She came around, realized she was wrong, and changed. But yeah, I can really see why a lot of people wouldn't like that.
When I first played XIII, I was 25 (damn, I feel old now), so my reaction to the characters were as such:
Lightning - "God, what a bitch. Stop punching people! Won't someone stand up to this bully?" *30 hours later* "Oh, she's not so bad, I guess. Who'd have thought I'd start liking a character more when they quit being a jerk to everyone!"
Hope - "Damn, poor kid... Seeing
must be rough at that age. I'd probably be pretty messed up if that happened to me at 14. Oh.. hmm, well now he's going a little bit crazy here. Lightning, why are you giving him a knife? Stop. Dang, this kid can make some awesome evil glares. Well, that's finally over. How the hell did he know about Oerba? wtf."
Vanille - "Ohh, gonna be one of THOSE types of characters. Gotta have one of those kinda ditzy/hyper girls, I guess. Oh, she's not too bad. She seems like she's hiding something behind all that happiness. She's kinda cute. Ok, I can't play this game around my roommate - the moans make me a little uncomfortable."
Fang - "Now SHE should be the main character! She's awesome. Love her voice, too. Aww, I shall ship VanillexFang in this game. It just makes sense!"
Sazh - "Now this guy is funny. Love the chocobo chick in his hair! WTF, LIGHTNING STOP PUNCHING HIM, JESUS. Lovin' this Vanille and Sazh storyline. Wow, awesome part of the story here! So.. now that's done, what is happening with Sazh now? Nothing? Oh..."
Snow - "Fuck off, Snow. Seriously."
Serah - "Serah...serah.... Who is that again? Oh, Lightning's sister. Damn, can you imagine how much it must have sucked to grow up with Lightning as your sister? Hope Lightning didn't beat her up too much as a kid."
I actually liked Serah more in XIII-2 when she wasn't just a plot device for the story (as in XIII). I thought she made for a decent main protagonist with Noel.
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There's a lot XII did well, but it's still one of my least favorite FFs tbh. At least in the grand scheme of the franchise. It's still a fantastic game, but I didn't really care about any of the characters (minus Ashe/Balthier), and the story was a total bore. It doesn't help that the game goes for such long periods of time with no story development whatsoever. The world was amazing, though. Battles were alright (although it felt like a worse version of FFXI's battle system tbh). I want to replay it eventually (Zodiac version pls) to see if my feelings have changed since when the game came out.