Vee is an Eevee that was experimented on by Team Rocket, thereby gaining the ability to freely change from an Eevee into any of his three Generation I evolutions (Vaporeon, Jolteon, and Flareon) and back.
Vee first appeared in Blame It on Eevee. When Red tried to challenge Erika, she stated that she wouldn't battle him unless he was able to capture a Pokémon of her choice, an Eevee. Red, with the help of Bill, tried to battle Eevee, but for every Pokémon that was sent out Eevee would turn into the Eeveelution that had a type advantage over it. This ability confused both of them at first, but Bill eventually figured out the ability, and Red managed to discover a way to keep Eevee from sensing the type of Pokémon sent out. Red was thus able to capture the Eevee. Realizing that his body had been experimented on to allow such an ability, Red took Eevee to the Celadon Gym and connected him to a healing machine to allow him to recover. Just then, Erika appeared once again, and feigned being a member of Team Rocket to force Red to fight for him. Despite Poli, Saur, and Pika's best efforts, Erika was able to counter their abilities one by one with her variety of movesets, and then approached Eevee, saying that if she disconnected him now, he would surely take his last breath. Red, evoking his deepest desire to defend the poor Pokémon, had Pika use a Substitute clone to distract Erika while having him guard the healing machine. Erika, impressed with their valiance, revealed herself to be an ally of the "good" Gym Leaders he had met before, and not only awarded Red the Rainbow Badge but also allowed him to keep Eevee, who Red nicknamed Vee.
In Clefabulous Clefable, Vee battled Bruno on Cerise Island, taking advantage of his ability to change into his evolutions, which was enhanced through the three Vermilion Harbor Evolution stones Red obtained from Giovanni. Vee starts by evolving into Flareon to counter Bruno's Hitmonchan's Ice Punch. He then evolves into Vaporeon to counter Hitmonchan's next move. Soon Poli and Bruno's Machamp also join the battle which seems even at first. Machamp then removes its belt and knocks both Vee and Poli off of the Onix battleground. Poli's Water Gun proves ineffective, but allows Vee to return via Acid Armor. Vee then evolves into Jolteon and proceeds to defeat Bruno with Pin Missile.
Vee was shown to have permanently evolved into Espeon prior to Forretress of Solitude when battling in Red's test to become Viridian City's Gym Leader. Vee went up against Porygon2 and was hit by its Zap Cannon, reducing his health below half. Red remained confident, and commanded Vee to use Morning Sun, followed by Psychic. This knocked Porygon2 out, winning Red the test. Red later explained to Sabrina that Vee had lost his ability to change into his Fire, Water, and Electric-type evolutions upon evolving into Espeon. Though he had now become a normal Espeon, Red still preferred him this way because he is no longer burdened by the sufferings of his past.
Due to being experimented on by Team Rocket, Vee developed a mistrust towards humans and would attack any that approached him. However, after seeing the lengths that Red was willing to go to protect him, Vee warmed up to Red and decided he wanted to stay with him. Vee is also a strong willed and brave Pokémon, shown by his willingness to evolve into his other forms to help Red despite it causing him pain. As an Eevee, Vee has shown the ability to sense a Pokémon's type through his ears. Vee would often use this to his advantage while battling by changing into a different evolution in order to have the type advantage.