This Water/Dark-type Pokémon is the fully evolved form of Froakie. It's just as fast and dangerous as any other ninja, and the throwing stars it can make out of water can shear metal. In Smash Bros., as well as being a graceful, speedy fighter, it can also use moves like Hydro Pump that let it travel through the air and avoid falling.
Greninja's Substitute down special lets you teleport to dodge attacks and then strike back, making it an effective and hard-to-predict way to counterattack. You can also control where you reappear by inputting directions. When you vanish, sometimes you'll be replaced by a log, other times by a cuddly toy.