Codes for Anime Fighting Simulator are released by the developers after it receives an update or when there's an upcoming event or a certain announcement is made regarding the game.
You should redeem these as soon as possible, because you'll never know when they could expire! If you want to use Anime Fighters Simulator codes, then follow the steps and redeem the codes.
You can also craft a shiny craftable fighter using five shiny mythical fighters and twenty shards.