Set to launch on December 7 for Nintendo Switch, Super Smash Bros. Ultimate will include every single fighter ever featured in the series’ nearly two-decade run, making it one of the biggest crossover events in gaming history. Fans can enjoy returning favorites like Pikachu, Pikachu Libre, Jigglypuff, Pichu, Lucario, Greninja and Pokémon Trainer, who fights by making commands to Squirtle, Ivysaur and Charizard. There are also numerous Poké Ball items that players can use to their advantage, including Mimikyu. According to the official Super Smash Bros. blog, Greninja turns into Ash-Greninja during its Final Smash:
A ninja-like fighter that battles with moves like the Water Shuriken. Greninja can use a log for clever counterattacks, and this Pokémon can even stick to walls. During its Final Smash, it turns into Ash-Greninja…!?
Source: Official Super Smash Bros. Blog