Lucas's Eevee

Lucas's Eevee is a Pokemon owned by Lucas from Pallet Town. It is best known as Lucas's main and partner Pokemon, and as the strongest Pokemon in his team. Unlike other Eevee, this Eevee is unable to evolve due to a unique genetic disorder, unseen in any other Pokemon before. By far, Eevee is also known for being powerful enough to defeat Legendary Pokemon (eg: Zekrom, Yveltal, etc.) despite his size.

This was Lucas's first Pokemon.

History
A day before Lucas was to receive his Starter Pokemon, Eevee was a wild Pokemon who lived with a group of seven other Eevee. On that day, Eevee and his friends were supposed to evolve into their Eeveelutions through their respective methods. While Eevee's friends evolved, Eevee was horrified when, no matter what method he used, he couldn't evolve. Believing that Eevee was inferior, his friends shunned him from the group.

Confused and rejected, Eevee ran off in a random direction, not knowing where to go. The next day, Eevee came across Pallet Town. Noticing a new trainer talking to a Professor in front of Route 1, Eevee approached out of curiosity. When the trainer (Lucas) saw the Eevee, he tried to run away, afraid that the wild Pokemon might attack him like normal wild Pokemon do from the tall grass. However, Professor Oak noticed that the Eevee was following Lucas out of curiosity. When the Professor told him this, Lucas decided to use the Premiere Ball his long-lost father gave him before disappearing to catch Eevee. However, Eevee, still feeling rejected and confused, refused to be caught, and ran off. When Eevee felt like he lost the two, he was about to eat an Oran Berry on the ground when a grumpy-looking Jigglypuff attacked him and used Dazzling Gleam. However, in the nick of time, Lucas, who had been following the Eevee all this time, protected the Eevee, although hurting himself in the process. Touched by Lucas's act of selflessness, Eevee fought back against the Jigglypuff and won. After following Lucas back home, Eevee let the new trainer catch him. However, much to Lucas's surprise, once Eevee was caught, he exited the Pokeball of his own will and refused to go back in. Professor Oak commented that it seemed like this Eevee had a similar personality to the main Pokemon of two other trainers.

When Professor Oak decided to check on Eevee for a bit to make sure he was fully healed, he noticed an irregularity in Eevee's scans. Upon further examination, he determined that it had a unique genetic disorder that completely disabled Eevee's evolving. However, this didn't deter Lucas. In fact, he considered it a challenge to train Eevee to his full potential. Proffesor Oak then remembered about giving Lucas his Starter, but Lucas declined, stating that he's already found his partner.

During their journey, Lucas and Eevee met countess new Pokemon and rivals, defeated many evil teams across regions, and even battled many legendary Pokemon and won. The only Pokemon, however, that Lucas and Eevee wishes to battle one day is Arceus. While Arceus has appeared to the two before, it always ended in Arceus telling the two that it will battle them "when they are truly ready."

Eventually, Eevee's old friends, and a Sylveon, tried to persuade Eevee to come back to them, impressed by his strength. However, Eevee declined, preferring to stay with Lucas.

Personality
Eevee has been shown to be a very determined Pokemon, despite it's inability to evolve. However, that has never stopped Eevee from becoming an extremely powerful Pokemon. Eevee has even earned praise from even the best Pokemon trainers and even Champions on how powerful he is. It is also shown that not only has Eevee come to terms with his inability to evolve, but actually embraces it. This is shown when N reveals that Eevee wants to represent it's own kind, proving that even a small Pokemon like Eevee can be powerful enough to defeat it's Eeveelutions. However, one flaw in Eevee's personality is that he refuses to go into his Premiere Ball, which makes Eevee open to dangers that a Pokeball might protect Eevee from. It also makes it hard on Lucas when he needs to Dynamax Eevee, which requires him going into his Premiere Ball.