In the Pokémon games
Greninja has only appeared in one Pokémon generation, the sixth one. It cannot be caught in the wild, making it obligatory to evolve Frogadier or trade.
In the competitive Pokémon scene, Greninja is top tier thanks to its speed and its Hidden Ability Protean, which makes it change its own type to the attack it is using, giving its attacking moves a boost.
In the Pokémon anime
Ash's Frogadier evolved into a Greninja in the episode A Festival of Decisions!, and is able to change into a special form called Ash-Greninja, which is a result of the strong bond between it and Ash. Ash released Greninja so that that it can protect Kalos with help from Squishy and Z2, the two Zygarde. Sanpei also owns a Greninja.
In Super Smash Bros.
Greninja appeared as a playable character in Super Smash Bros. 4 and Super Smash Bros. Ultimate. It is speedy and can use Water Shuriken as well as fight up close.