Mega Man X Dive is managing to straddle the line between "legitimately fun" and "intentionally frustrating gacha game designed to rob people of their money." On one hand, it plays just like a Mega Man X game, and while the touch screen controls suck, it's possible to pair one's phone to a PS4 controller and map the controls. Which makes it surprisingly fun for a mobile game. On the other hand, the game relies heavily on leveling and stats, with the level difficulty quickly outpacing the rate at which your character grows. Grinding is awful, as levels provide minimal experience points after the first time they are beaten, and you can only skip dive for easy experience so many times before the game cuts you off. And while it is possible to beat a level without being at the recommended strength, enemies take longer and longer to kill, while their own attacks increase in power until they can eventually one-shot you. Additional power can be squeezed out a number of ways: leveling up weapons, upgrading weapons, ranking up the characters, equipping cards, leveling up skills, etc. But doing all that requires items, which you can gain from beating levels or, you guessed it, buying at the store.
Unlocking characters can be really easy or really hard, depending on the gacha. You get one free capsule roll a day, which can get you either a character or a weapon. And unless you're just starting, it's probably going to be something you already have. After that initial roll, additional rolls require metals. Which you can earn by accomplishing acheivements, or by buying them with real money. On the plus side, if you get a character or weapon you already have, you get points that you can redeem at the store for character or weapon memories, which can be used to rank up a character or weapon. On the down side, it takes a fair number of memories to rank up a character or weapon. Way more than you get from a duplicate. But that's still better than nothing.
Character-wise, you start with just X. Axl and Zero can be picked up in story mode, although if you use the gacha, you'll probably get them before that point. Alia is apparently being given out to new players who login 7 days in a row. As far as I can tell, everyone else comes from the gacha.
While there are lots of characters, they all play similarly, with all of them having access to the same weapons. That said, they do have different skills, and their base stats are different enough that playing different characters actually does feel like playing a different character.
Some of the characters also come with unlockable skins, which change their appearances and can give stat bonuses. Thus far, I've only managed to unlock X's command mission costume.