Power Ninja is one of the three skins released under the Final Attack part of the Anime (Volume II) offer. It could be purchased as part of an item deal in early November 2017.
Power Ninja could be purchased for $9.99 in a limited time deal that included 16000 coins. It was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: The Final Attack part of the Anime offer was released again for a limited time in late August 2020. For this release Power Ninja and 4200 coins could be purchased for $4.99.