Sylveon is the newest evolution of Eevee to be introduced to the Pokémon series. It was first featured in Pokémon X and Y where you had to level up Eevee while its affection towards you is high, after which it evolved into the Intertwining Pokémon, Sylveon. The Fairy-type Pokémon has gone on to be pretty popular, going as far as being the only evolution of Eevee to be featured in Pokémon Unite. Despite this, it’s not present in the newly released Generation IV remakes, Pokémon Brilliant Diamond and Shining Pearl.
This likely has to do with Sylveon not being introduced until two generations after the trio of Diamond, Pearl, and Platinum were released. Brilliant Diamond and Shining Pearl are mostly straightforward remakes of the original titles, and are notably lacking features that originated in later generations, including Dynamaxing and Mega Evolutions.
Interestingly enough, Sylveon didn’t make the cut, but its typing did. Fairy-type Pokémon, which did not exist prior to Pokémon X and Y, can be found in-game. It seems like if Sylveon was omitted due to not being present in the original game, that its typing would be as well, but that isn’t the case, making the question of why Sylveon is missing a rather confusing one.
