To unlock the Inverse Mode in Vampire Survivors, players must obtain a Relic known as Gracia's Mirror. Survivors can get this Relic in the Eudaimonia Machine, a hidden stage that becomes accessible once you have obtained all of the standard Stage Relics.
Just defeat the boss on the 5th stage and you will be offered 2 options pick "too easy" and you will unlock Gracia's mirror.
Select Gallo tower and tick inverse at the bottom.