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Eevee is a small, mammalian, quadrupedal Pokémon with primarily brown fur. The tip of its bushy tail and its large furry collar are cream-colored. It has short, slender legs with three small toes and a pink paw pad on each foot. Eevee has brown eyes, long pointed ears with dark brown interiors, and a small black nose.

Eevee is rarely found in the wild and is mostly only found in cities and towns. It is said to have an irregularly shaped genetic structure that is easily influenced by its environment. This allows it to adapt to a variety of habitats by evolving. Eevee can potentially evolve into eight different evolutions. Eevee can also start to adopt the face of the Trainer that owns it. Eevee's genes are believed to have the key to solving the mysteries of Pokémon evolution. It's shown in the anime that once an Eevee evolves into one of its eight evolved forms, their evolution cannot be changed especially with an evolution stone.

Forms
Eevee has two forms: the partner Eevee and Gigantamax Eevee.

The partner Eevee has purple eyes and a lighter shade of fur. These design elements were likely taken from the anime, which implemented similar traits starting in Pokémon the Series: Ruby and Sapphire. The partner Eevee has higher base stats and access to moves that normal Eevee do not. The marking on the tip of a female partner Eevee's tail is flower-shaped. Before Pokémon Sword and Shield, this trait was unique and wasn't found on other female Eevee. Since Sword and Shield, however, the unique tail pattern is present on all female Eevee as a true gender difference. As a result, Pokémon GO was updated to apply this change to all female Eevee as well. The partner Eevee is the only known Pokémon capable of learning the moves Bouncy Bubble, Buzzy Buzz, Sizzly Slide, Glitzy Glow, Baddy Bad, Sappy Seed, Freezy Frost, and Sparkly Swirl, as well as the partner power Veevee Volley.

As Gigantamax Eevee, it has become larger and more fluffy. The fur collar becomes longer with several triangular furs pointing straight up. The right ear has three red clouds circling it. Due to the fur becoming fluffier and more luxurious, any enemies caught in it will become enamored and lose the will to fight. Eevee becomes even more playful and rambunctious after Gigantamaxing, desiring to play with its opponent and often unintentionally crushing them with its size.

Evolution
Eevee can evolve into eight different Pokémon (collectively called Eeveelutions).

(Source: Bulbapedia)