Greninja is a dual-typed Water/Dark-type frog Pokémon which is themed after ninjas and also known as the "Ninja Pokémon", its species title. It was introduced to the Pokémon series in its sixth generation, in its flagship games Pokémon X and Y, and is the final evolved form of the starter Pokémon Froakie, evolving from its middle stage Frogadier at level 36. According to the Pokédex and based on its stats in the Pokémon games, Greninja is extremely fast (in fact, it has the highest raw Speed stat of any Water-type Pokémon or fully-evolved starter Pokémon) and relies on this speed in battle, confounding its prey with swift, ninja-like grace before striking with its unique Water Shuriken technique, said to be able to shear metal. In exchange, however, it is rather frail and has low defenses.
In the Pokémon games, Greninja has access to numerous unique abilities. In addition to the Water Shuriken move, which it uses in Super Smash Bros., it is also the only Pokémon able to learn the ninja-derived move Mat Block, where it flips a tatami mat in front of it to shield itself and its allies from incoming attacks. In addition to these two unique moves, it has access to a diverse range of offensive and status moves; it can even uniquely take advantage of them with its rare hidden ability Protean, which changes its type to match each move it uses and ensures it gets the maximum possible power out of every attack. Its prowess in Pokémon battles resulting from Protean has led to Greninja being considered an extremely powerful and useful Pokémon by the competitive Pokémon scene, and some major scene bodies even believe it to be an "Uber-tier" Pokémon in the same vein as Mewtwo.
Greninja debuts as a newcomer playable character in Super Smash Bros. 4, first revealed in the Super Smash Bros. Direct on April 8, 2014 alongside Charizard. Fitting with its ninja motif, Greninja is a quick, mobile and somewhat weak-hitting fighter, albeit not to the extent of Sheik, and possesses special moves that can catch opponents off guard, such as Shadow Sneak and Substitute.