Sylveon is one of the many evolutions Eevee can become given you follow the exact steps you need to have to turn into that particular evolved form in the Pokémon games. In Pokémon Go, you have to follow similar intricate steps, but these steps are different because Pokémon Go and the traditional Pokémon games do not have the same items. Of the eight Eevee evolutions, Sylveon is not available in Pokémon Go.
Sylveon is a Fairy-type Pokémon, meaning its one of the rare Pokémon types that are super effective against a Dragon-type. It first appeared during Pokémon X and Y versions set in the Kalos region. At the time of this writing, Pokémon Go is still introducing Pokémon from the Unova region, which has Pokémon from the Black and White versions. Sylveon will likely not be available until the Kalos region makes an official appearance.
The traditional method of how you evolved an Eevee into a Sylveon involves you obtaining a high friendship with it, which you earn by battling with and having it in your party. Evolving Eevee into Sylveon in Pokémon Go will work differently, but it might seem the same way. Based on the current mechanics of the game work, you will likely have to level up Eevee’s buddy level and walk around with it to receive a Sylveon. This information is unconfirmed and pure speculation.
We’ll be updating this guide when Sylveon does become readily available in Pokémon Go. We could see its release sooner before the Kalos region becomes available. The developer Niantic has been know to release Pokémon before their region officially releases, such as Galarian Pokémon being made available through limited-time events.