Previously, Kirby: Battle Royale held a poll of all the Copy Abilities, and Mirror was the winner. Mirror was announced to be added to the game in a post-release update, and the Kirby 25th Anniversary Twitter account announced that Mirror will be available this week in Japan.
The update has not been announced for Europe, and the US, where the game has yet to be released. Meanwhile, the second Copy Ability poll has ended, and the winner will be announced in January 2018.
Kirby: Battle Royale will be available on Nintendo 3DS starting January 19, 2018.