What is a Reversion Card?
A reversion card is a card used to reset a character to level 1 and return all used upgrade materials to the bag. A reversion card is often called a reset card.
How to get a reversion card / reset card
From what I know, there are only a few ways to get a reversion card, namely by buying at the mall.
Level up rewards
Reset cards can be obtained from level up prizes
Free reset card from Event
You can also get a reversion card from the event prize