I'm not sure specifically which 'A Sad Eevees Story' you mean. It could be about an Eevee in a Pokémon world that has lost its trainer, or maybe it was bullied by other Pokémon. Without more details, it's hard to say exactly.
Since it's a fanfic about two Eevees, it might revolve around their evolution. Perhaps one Eevee has a strong desire to evolve into a certain form, like a Vaporeon because it loves the water, while the other Eevee is content with being just an Eevee for a while. The story could detail their journey towards or away from evolution. It could also include elements of the Pokemon world such as trainers, battles, and the search for rare items. There might be conflicts like rivalries with other Pokemon or misunderstandings between the two Eevees that they need to overcome.
Given that it's 'a tale of two eevees fanfic', the two Eevees are probably the main characters. However, there could also be other Pokemon or trainers that play significant roles in the story. Maybe there's a friendly Pikachu that helps the Eevees out in times of trouble, or a strict trainer who has an influence on their development.
Well, perhaps Naruto is sent to a dimension where Eevees are the main inhabitants. He has to adapt to living with them. He could form strong bonds with individual Eevees, like an Eevee that's really brave and they go on adventures together, exploring the new land and facing challenges that are unique to this world of Eevees.
Obviously Naruto is one of the main characters. And then, there would be the Eevees. There could be a special Eevee that stands out, like an Eevee that is more curious about Naruto than the others and becomes his closest companion among the Eevees.
To write a really sad story, start with a character facing a profound loss or heartbreak. Build up the emotions gradually, describe the details of their pain, and show how it affects their life.
One possible story could be that a trainer has too many Eevees due to some unexpected breeding or a special event. They can't take care of all of them, so they put them up for adoption. Another idea is that a Pokemon rescue center has a box full of Eevees that were found abandoned in the wild, perhaps because their previous trainer couldn't handle their care anymore.
My '5 words sad story' is 'Hope, gone, dark, fear, end'. A person had hope, perhaps for a better future or for a relationship. But then it was gone. All they could see was darkness. Fear crept in and they felt like it was the end of everything, as if there was no way out of their misery.