Vee with his temporary evolutions
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 You Know...Articuno!, Vee and Red's other Pokémon battled Team Rocket members who were pursuing Articuno in the Seafoam Islands, a task which they ultimately failed. Soon afterwards, Red sent Vee into storage after restoring Aero in Cinnabar Island. It was later discovered that when Red had sent him to Professor Oak, Vee had been captured by Team Rocket again. In The Winged Legends Red rescued Vee after Sabrina had no use for him, as she had now obtained the merged legendary birds.
As revealed in Quilava Quandary, Vee was a part of the team Red used during the Pokémon League.
Vee battling Bruno's Hitmonchan
The Evolution Stones Vee used to evolve
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 an Ice Punch from Bruno's Hitmonchan. 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 stamina 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.
In The Last Battle X, Vee had a quick battle against Bruno and his Tyrogue. Vee was able to counterattack his use of Double Team by using Psych Up before attempting to flee from the fight.
Vee appeared in a flashback in Secrets from Sneasel, when Yellow recalled how Red had taken the test to become Viridian City's Gym Leader.
Vee was last seen in Epilogue interacting with Ditty in the Battle Frontier following Guile Hideout's defeat.
Vee learning a Pokémon's type by swiveling his ears
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. After Gyara joined the team, Vee was the only one shown to not be afraid of him, due to having suffered similar torture under Team Rocket. When Gyara lashed out, Vee was the first to try and help and motivated the others to join him.
While an Eevee, he had the unique ability to evolve into a Vaporeon, Jolteon, or Flareon due to Team Rocket's experiments, which helped it maintain a type-advantage regardless of the situation. Despite this, changing forms brought him pain, though he was willing to endure it to help Red out. After permanently evolving into Espeon, however, this ability was lost. Both Vee and Red were okay with it, as this allowed Vee to put his past with Team Rocket behind him.
Vee has shown the ability to sense a Pokémon's type through his ears and would use this to his advantage by changing into a different evolution in order to have the type advantage.