Greninja is a bipedal, frog-like Pokémon. It is mostly dark blue with a yellow chest, a white triangular marking over each eye, a light blue four-pointed star on each thigh, and yellow on the lower half of its face. It has red eyes with white pupils and its mouth is hidden behind a large, pink tongue that wraps around its neck and extends outward behind its head. Running down the middle of its head is a fin-like extension, and there is a similar fin on each side of its head. Light blue webbing connects its head fins. There is a large, white bubble-like bump on each elbow and knee. Its back feet have two toes, while its front feet have three toes. Each digit has a bulbous tip and yellow webbing.

Greninja moves with the speed and grace of a ninja. It uses swift movements to confuse its enemies while it slices them up with weapons made of compressed water, most commonly in the form of throwing stars. These throwing stars are sharp enough to split metal.

Forms
Greninja has an alternate form: Ash-Greninja

Through a process called the Bond Phenomenon, certain Greninja, such as Ash's Greninja in the anime, can change into an alternate form called Ash-Greninja. In this form, Greninja's skin turns a lighter shade of blue, while its face gains red stripes and its cheeks and side fins turn black, changing shape to resemble Ash's hair. Its legs are stronger in this form, and it gains the ability to form large Water Shurikens on its back. Ash-Greninja is always male.

Evolution
Greninja evolves from Frogadier and is the final form of Froakie.

(Source: Bulbapedia)