How to reroll - Final Fantasy Brave Exvius Wiki
That's a good general guide, although fair warning this is a *terrible* banner to start the game on because the TMRs are mediocre and the units are awful, so you'll have to reroll even longer to get a good starter. At a bare minimum you want to reroll until you get an account that starts with Lightning, Luneth, Gilgamesh, or Noctis. Lightning and Luneth are basically ezmode starter units because they innately learn dual wield which is the strongest ability in the game by far. Gilgamesh is a strong unit who eventually gives you an accessory that grants dual wield, and Noctis is just an extremely versatile and powerful unit. Ideally you'll also get some good support units as well but getting one of those 4 premium units makes a really big difference. Rerolling is *SUPER* tedious, but this is your one and only chance to guarantee yourself a powerful unit.
Basically you start a new account, play up until it gives you the initial batch of five free rare tickets, cash in all the summon tickets + your daily discount pull, and if you don't get one of the mentioned heroes you wipe that data via the steps in that guide I linked.
A) Start new account, use your free tickets, decide it's not worth keeping, link this Exvius account to a dummy Facebook account by logging in to facebook. Menu -> options - Login to facebook. Then from the exact same spot, log out of facebook.
B) You'll be back at the original menu. Start new account. This account is saved locally on your device. Use your free tickets, nothing worth keeping, so you're gonna reroll. While in game you go to the menu and login to facebook, then enter the info from your dummy FB account. This will retrieve the Exvius account linked to your dummy FB account, overwriting the locally saved account.
C) goto B
You don't have to link the Exvius account you're keeping to a facebook account, but there are some benefits. The only way to transfer your data between devices (like your phone and an emulator) is with it linked to facebook.
I do have some better starter accounts than that Gilgamesh/Luka account I offered, but they aren't free and I doubt you're in the 'pay real moneys' stage yet lol
*edit*
When you do reroll, I honestly think you'd have better luck rolling on the generic rare summon banner rather than one of the two rate up banners.
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Normally that's a giant no-no because if the event banners suck you just don't pull, but when you're rerolling for free the inefficiency of pulling on that banner doesn't matter and you'll actually have a slightly higher chance of getting one of the premium units I mentioned, instead of the elevated chance of getting Ace or Dark Fina on their respective banners.