If you are satisfied with your account after rerolling, you can save your account by setting up a data transfer information. In the game's menu, scroll all the way down and tap the "Data Transfer" option on the left.
Tap "Data Transfer Setup", then write down a password that you can remember and select OK.
Once you're done, remember to take a screenshot of it, or write the information down so that you don't forget. It's also recommended to write down your player ID that you can find in the "Profile" from the menu or title screen in case you need to provide more information about your account for support.
How to Transfer Data
If you want to transfer to a different device, go to the title screen and tap "User Support" on the bottom right, and tap "Data Transfer". Then, simply put in the Data Transfer ID and Password in.
Additionally, you can also use your email to link/connect to Square Enix Bridge. From the menu, scroll down to "Email Registration" that's right next to "Data Transfer".
This has the option to connect to an account on other platforms that you may have, or you can register to make a new account. Square Enix Bridge can provide more security for your account since it can use software authenticator, as well as other features.