How to hoard black keys

--Requires Root--
There's no way to fuck this up unless you do something horribly, horribly wrong but you might want to backup your stuff before doing any of this in case something bad happens, you have been warned.

Step one:
Download or install Titanium Backup PRO (Might work with Android 5.1 or phones that support multiple profiles without having to use Titanium backup)
MEGA link: chrome://mega/content/secure.html#!d4hmFAYT!l1eKRRvxfMF31UKu16FFuVM73ObI_XSAVGT8qFUv4RM

Step two:
Make a "Main" profile, your current Fate/grand order account will be bound to this profile after completing the next step.

Step three:
Set up Fate/Grand order as a multi-profile application.
The Puzzle and Dragons General has a very handy useful guide for this
http://i.imgur.com/2FUHexy.png

Step four:
After you have your Main profile and have set Fate/Grand order as a multi-profile application, create a new profile and launch Fate/Grand order.
If you have done it correctly, the game will launch like its your first time running it and will attempt to download additional files.
If your connection is fast enough to download 130~ MBs of additional data in no time then let it download, if you have a slow connection or just don't want to re-download the files then proceed to next step.

Step 5:
Using your file explorer, navigate to "/data/data/com.aniplex.fategrandorder/.data_profiles(This is a hidden folder)/(Name of your profile)/Files" and copy the three big folders you'll find there to your current profile located at "/data/data/com.aniplex.fategrandorder/". Replace everything if prompted and delete authsave, authsave2, signup save and other similar files there like you're doing a reroll.

Final step:
Launch Fate/Grand order, the game should launch you directly to the title screen and you're free to play normally.

If you ever want to switch back to your first profile, go to Titanium backup and switch profiles.
After switching profiles, start up fate grand/order and it should launch your account. Now you can switch between alts with a few taps on the screen.

Using this method, you'll need around 130 mbs~ of space for every account you want to have on your device so you can have as many alts as your phone allows.