What is the legality of Pokemon showdown? [closed]

Pokemon Showdown uses images, sprites, music and other Intellectual Property (IP) of Gamefreak, Nintendo & The Pokemon Company. It is most likely breaching some of Nintendo's copyright.

Note that Showdown and other sites don't need to be making money in order to be infringing. Copyright infringement occurs regardless of whether there is profit being made. Usually, the 'takedowns' people hear about are because the copyright holder believes that the infringing material is taking away from their profits. If Showdown was providing enough of an incentive to play on their service that people stopped buying the actual games, it's likely that Nintendo would act on this.

Note that the inaction on Nintendo/Gamefreak's behalf so far does not constitute their blessing. At any point they can choose whether to act on a case of copyright infringement. If they wanted, they could take down or severely limit the capabilities of fan sites like Pokemon Showdown, Serebii, Bulbapedia etc for using their copyrighted materials.

So why don't they just issue takedowns on everything? It's bad for business. There is a beneficial effect of having fans build sites, curate libraries of resources and constantly talk about and consume your content. It's free marketing. Whether a copyright holder chooses to exercise their right is essentially a cost/benefit analysis.

In conclusion, Showdown most likely infringes on Nintendo's copyright. However, as of writing, Nintendo has not acted on these potential infringements. While this can change at any point, there is no danger to using the services while they are available.

_{Also, show your support to Nintendo/Gamefreak and buy the actual Pokemon Games! :)}