Pokémon Brilliant Diamond and Shining pearls retains some elements of the original fourth generation games, but one aspect that has changed is the inclusion of Fairies, no matter how limited they are.
This means that Pokémon like Ralts, Mr. Mime and a few others retained their newer typologies from later generations-Diamond and Pearl, giving them a leg up on some Pokémon they might have struggled to fight off.
Of those Fairy types, Ralts is the most hunted, both because of its evolutionary lineage and because it can evolve into two separate Pokémon of different types depending on what you need on the team – Gardevoir or Gallade.
Gardevoir retains its Fairy/Fairy roots, acting as a particularly powerful Attacker that can dominate multiple opponents. Meanwhile, Gallade was introduced in Diamond and Pearl, gains Psychic/Fighting entry and acts as a full-blown attack option.
Locating and capturing a Ralts can be difficult as Pokémon can use Teleport to flee battle after a single turn. Also, because BDSP based on the original game and not Platinum, you cannot find Ralts in the wild.
Instead, you’ll need to walk through the Grand Underground and find Fountainspring Cave, Dazzling Cave, and Stargleam Cavern Pokémon Hideways to encounter one as a rare creature. You may also meet Kirlia under certain circumstances.
Ralts evolves into Kirlia at level 20 and again into Gardevoir at level 30 without using any other items or methods. To get a Gallade will require you to use the Dawn Stone on a male Kirlia. You can find the Dawn Stone on Route 225, inside Mount Coronet, and in the Grand Underground.
https://dotesports.com/pokemon/news/how-to-catch-and-evolve-ralts-in-pokemon-brilliant-diamond-and-shining-pearl How to catch and evolve Ralts in Pokémon Brilliant Diamond and Shining Pearl