For a plot, Eevee might be left behind when its trainer goes on an adventure without realizing. Eevee whimpers as it watches the trainer leave. It then decides to go on its own adventure to find the trainer. Along the way, it meets different Pokemon species and has various challenges. It could also be that Eevee is at a Pokemon center where it sees other sick and injured Pokemon. The sight makes it whimper, and it tries to comfort the others. This leads to it making new friends and learning about the importance of kindness in the Pokemon world.
Effective writing of Eevee's character when it's whimpering involves several aspects. Describe its whimpers vividly, like the pitch and how often they come. Eevee's emotions should be clear, whether it's fear, loneliness, or pain. Since Eevee is a small and often underestimated Pokemon, use that to your advantage. When it whimpers, it can be a moment to show its inner strength as well. Maybe it whimpers but then decides to be brave and face the situation. Additionally, consider how Eevee's environment affects it. If it's in a cold place and whimpering, it might seek warmth and show how adaptable it can be by trying to find a cozy spot or even using its own fur to keep warm while still whimpering a bit.
One idea could be Eevee going on a journey to discover its true evolution form among its friends. Another could be the friends teaming up to save a rare Pokemon in their habitat. Maybe Eevee and its friends get involved in a Pokemon contest and face various challenges.
One idea could be an Eevee that is born with a rare genetic mutation that gives it a new, unheard - of evolution type. The story could follow its journey as it tries to figure out this new power while facing both friends and foes who either want to help or take advantage of it.
One idea could be exploring the different evolutions of Eevee within a family setting. For example, a young trainer has an Eevee family. One Eevee evolves into Vaporeon because it loves to swim in a nearby lake. Another might evolve into Flareon due to its fiery temper. This can lead to a story about how the family copes with the different personalities and abilities of each evolved form.
A really interesting plot might be that the Eevee is actually a reincarnation of a great Pokemon hero from the past. The trainer discovers this as they journey together and face various challenges. This knowledge makes them a target for an evil organization that wants to use the Eevee's power for their own malicious purposes. Along the way, the Eevee and the trainer meet other Pokemon and trainers who either help or hinder them in their quest to protect the Eevee's secret and fulfill its destiny.
One idea could be that the male and female Eevee are on an adventure together and they discover a magical place that makes their bond stronger. They could face challenges together and their relationship deepens as they rely on each other.
One idea could be an Eevee that wakes up in the mystery dungeon world with no memory of its past but with strange powers. It then has to go on a journey to discover its true identity and where these powers come from. Another could be an Eevee that forms an unlikely alliance with a group of Pokemon from different species to explore a newly discovered and extremely dangerous mystery dungeon.
Well, it could be that the Arcanine is injured. Maybe it got into a tough battle with a powerful opponent in the fanfiction world and is in pain, thus whimpering.
Maybe in the fanfiction, Eevee and Vulpix are both lost in a mysterious forest. They start to look for a way out together. Along the way, they come across ancient Pokémon ruins. Inside the ruins, there are puzzles and traps. Eevee, being a very adaptable Pokémon, uses its different forms' abilities (even if it doesn't actually change forms yet in the story) to solve some of the puzzles. Vulpix uses its fire to light the way and keep away any dangerous Pokémon that might lurk in the dark corners of the ruins. As they progress, they become closer friends and finally find their way out of the forest.
One plot idea could be that the male reader discovers an injured female Eevee in the forest. He takes her in and nurses her back to health. As she recovers, they form a strong bond, and the Eevee decides to stay with the reader instead of going back to the wild. Along the way, they face some challenges from other Pokémon or humans who don't understand their relationship.