For a long time (6 games in a row, going from FFIV to FFIX) the Final Fantasy games used a combat system called ATB (Active Time Battle) where characters and enemies would charge up a meter in real time and could then take actions when their meter filled up. While that action was going on, or while you were thinking about what to do with that action, or while you were manually entering the commands to take the action everyone else's meters would keep charging up. In theory this was a system that was designed to make character speed matter (the older games gave each character exactly one action per round and speed only determined order in the round if anything at all) but in practice everyone's bars would just fill up while you were entering in commands anyway. Unless you played on a slow speed I guess, or are really fast at entering commands, or cheesed the system out by pretending to use an item since the ATB bars would pause when using an item in some games. I liked the idea of the ATB system, and liked it when I was a kid, but when you really dig into how it works it gets a little disappointing.
Final Fantasy X switched it up and went to a system more like Final Fantasy Tactics. In FFX the game pauses when it's someone's turn to take an action. You can use all the time you want to think about what you want to do, or to enter the command in, or to doze off. That happened to me the other day and in an ATB game it would have meant game over. Here the game just assumed I was thinking really hard about what to do with Kimahri. (What am I going to do with Kimahri?!?) When I woke up I was able to finish the fight, meander to a save point, and go to bed. Perfect!
But I digress... The system works by assigning every action in the game a number from 1 to 8 indicating how fast it is. Then you multiply that number by a value determined by your agility. That's how many ticks will pass before you get your next action. Monsters work the same way. The game has a nice list ordering all of the upcoming turns so you can see who is going to go next and what the ordering will be assuming everyone takes a standard speed action. I'd assumed agility was going to be a linear relationship with how often you get to go but it's really, really not. Early on the steps are in increments of 1 or 2 agility but the later steps are huge. There's no difference between 44 agility and 61 agility, but that 62nd agility gives you 20% more actions! The 98th agility is worth 25% more actions and the 170th agility is worth 33% more actions! But is it really worth spending all the levels to go from 98 to 170? Even if every node was a 4 agility node that would be 18 levels worth of agility, and they're not all going to be 4 agility nodes until you've reached the point where you're destroying the monster arena. Aiming for 44 or 62 seems pretty reasonable but if Rikku has hit those breakpoints she really needs to teleport out of her agility area and go find some strength or something! She can probably spend 18 levels on strength and end up doing more than 33% more damage per action...
There's one other aspect to the CTB system used in FFX, and that's the ability to tag in characters from the bench. Any time it's your action you can swap out the current character for one of the four characters not currently on the field. They come in and get to immediately take an action. Anyone who participated in the fight gets full experience too, so there's an incentive to tag in characters to do something, anything, in order to keep leveling up. This also let the game design monsters that are really hard to deal with, with one critical weakness, since you're guaranteed to have that tool in your toolbox. There are robots that are pretty tough, except if Rikku steals from them they instantly die. If this was a normal Final Fantasy game and there was a chance Rikku wasn't in your party this would be a bad design, but here it's awesome. It always feels like there's a reason to use each person, which makes you use your whole team for the whole game, which is a really different change of pace.
Everyone except Kimahri. All 6 other characters have something they do best. (Tidus hits evasive ground monsters, Auron pierces heavily armoured monsters, Rikku kills robots, Wakka hit evasive flying monsters, Lulu casts elemental spells on super tough monsters, and Yuna is the healer and can summon in aeons to deal with tricky fights.) Kimahri has no niche. He's the jack of all trades in a game where you can make the hot tag to the expert in every situation. He's the second best at piercing armour, sure... But when Auron one shots the enemy why do we need the second best one? I frequently send him down the black mage area since he starts with an ability that restores his mana, but then he's strictly worse than Lulu for pretty much the entire game. Maybe you make him into a healer? He's terrible compared to Yuna though, and do you really need two healers in a three person group? (It actually can help to have a second person with the life spell, but eventually you get some white magic spheres so that person could be anyone!)
I still use Kimahri. He needs experience because he never gets tagged in to do anything and I feel bad for the guy. And in a normal system he'd probably be the best character! He can do a little bit of everything so he'd be able to deal with the huge variety of problems the random encounters can throw at you. But the tag in system obsoletes the jack of all trades and that makes Kimahri sad. Don't worry Kimahri. I'll still use you.