Known as the Evolution Pokémon, Eevee is a fennec-like Pokémon with an unstable genetic code, which allows it to evolve into seven different Pokémon depending on the situation. Three of these evolutions are the result of Eevee being exposed to either a Fire Stone, Thunderstone or Water Stone. Two more of the evolutions involve maximum affection with the Trainer and accordance to the time of day. The final two evolutions are the result of training in specific areas.

Eevee is a mammalian creature with brown fur, a bushy tail that has a cream-colored tip, and a furry collar that is also cream-colored. Eevee has brown eyes, big ears, and pink paw pads. Eevee is said to have an irregularly shaped genetic structure, enabling it to evolve into multiple Pokémon. Eevee are quite rare, but are able to live almost anywhere, as they may evolve to suit their surroundings.

While appearing very simple in appearance, Eevee is best known for its unstable genetic code which allows it to change and mutate into different forms (through evolution) depending on its environment. As a result of its evolution, Eevee would become better suited to living in its surrounding environment. All of Eevee's evolutions are of unique typing in comparison to each other. In Pokémon Red and Green, the player was able to evolve his or her Eevee into one its first three evolutions (Vaporeon, Jolteon, or Flareon) by the use of an elemental stone (though only one can be obtained at a time). Starting from Pokémon Gold and Silver, the player could further evolve his or her Eevee into either an Espeon or an Umbreon by making the Eevee to achieve maximum happiness (though the evolution depended on what time of day it was as Eevee evolved into Espeon at daytime while Eevee evolved into Umbreon at night). The final two evolutions of Eevee were obtainable upon the release of Pokémon Diamond and Pearl. Leafeon, Eevee's grass type evolution is obtainable upon leveling up near a moss rock, meanwhile Eevee's ice type evolution, Glaceon, is achieved by leveling up near an ice rock. These rocks are found in the Sinnoh and Unova regions.