Jolteon, via Thunder Stone Flareon, via Fire Stone Espeon, via max Friendship and leveling up in the daytime Umbreon, via max Friendship and leveling up at night Leafeon, via Leaf Stone Glaceon, via Ice Stone Sylveon, via max Friendship and leveling up knowing a Fairy move
Eevee is a Normal-typePokémon, extremely famous for the loads of evolutions it has. While not a "true" mascot for the series, it IS one of the more commonly-seen Pokémon in fanfics; unfortunately, owning an Eevee is often the stereotype of a "Mary Sue" character.
In Pokémon Yellow, Blue's starter is an Eevee. This further evolved in the game's 2018 Switch remakes to Eevee potentially being your Starter in Let's Go Eevee. This special Eevee can't evolve and has a unique appearance as well as different stats; because it's a female Eevee, the unique appearance would go on in Generation 8 to become the default appearance for all female Eevees, making it one of a few rare Pokémon who did not originally have a gender difference to get one. This may also explain why Gary had an Umbreon in the animé.
Eevee is also a Starter in Pokémon XD: Gale of Darkness (Espeon and Umbreon are Starters in its prequel).
Eevee appeared in Super Smash Stadium as a Poké-Ball Pokémon, functioning much like it did in its canon Smash appearances. However, because it debuted alongside Pong, it didn't last very long. The reveal trailers for the video game claim that it was a "cut" Pokémon.
All canon Pokémon appear in Tropes vs. Women in Pokémon Training, including Eevee. This game brings back its old evolution methods for Leafeon, Glaceon, and Sylveon, with the Moss Rock appearing in Dan Meadows and the Ice Rock appearing in Snowside City. In addition, Eevee has the following new evolutions:
Manteon, by leveling up while holding an Insect Plate or Silver Powder
Medieveon, by leveling up while holding a Draco Plate or Dragon Fang
Aureon, by leveling up while holding a Fist Plate or Black Belt
Aereon, by leveling up while holding a Sky Plate or Sharp Beak
Phobeon, by leveling up while holding a Spooky Plate or Spell Tag
Terreon, by leveling up while holding an Earth Plate or Soft Sand