It appears and vanishes with a ninja’s grace. It toys with its enemies using swift movements, while slicing them with throwing stars of sharpest water.
– Pokémon Y Pokédex
Greninja is probably the most popular of the Gen 7 starters, considering it’s prominently featured in the anime, and being a playable character in Smash Bros. It won a poll in Japan to decide which Pokémon would be distributed at the Volcanion movie.
It’s also a popular Pokemon competitively due to its speed and Sp Attack. Its hidden ability, Protean, lets it change type based on the move it’s using, which means it always gets STAB on its attacks.
In the anime, Ash’s Greninja does this merge thing with Ash that’s not really mega evolution. At first Ash-Greninja was added to spinoff games like Pokemon Shuffle or Pokemon Picross, but in the gen 7 demo, it has become an official Pokemon form in the main games, through its ability Battle Bond.