Rerolling is a way to get the rarest characters in games which has gacha system. Players have to spend gems to roll a gacha, and characters are randomly distributed. Most of those games allow a player to roll a gacha without charge or by less cost at first time and that happens after tutorial. If players cannot get a best character at that time, they uninstall the app to try to roll the bonus gacha again.
The most efficient rerolling
Install the app.
Finish the tutorial 1.
Finish tutorial 2 and receive presents.
Finish tutorial about enhancing and awakening.
Roll Gacha.
Play multiplay mode to get more than 30 Ninja Pearls.
Conduct Running Start Summon ( multi Gacha ) after obtaining enough gems.
If a character you want to get appears, finish rerolling. If not, uninstall the app and repeat from step 1.
It would just save you more gems if you create another account. It's your choice, you can choose not to make another account and just keep earning gems for the next re-roll