Kirlia is one of the many Pokémon you can catch in Brilliant Diamond and Brilliant Pearl. Initially, you evolved Ralts into Kirlia to receive a Gardevoir as the original final evolution. However, in Pokémon Brilliant Diamond and Brilliant Pearl, you will have the option to evolve it into Gallade, a Psychic and Fighting-type Pokémon. Here's what you need to do to evolve Kirlia into Gallade into Pokémon Brilliant Diamond and Brilliant Pearl.
The first thing you need to do is make sure you catch a male Ralts. If you don't catch a male Ralts, you can't become a male Kirlia. A female Kirlia cannot evolve into Gallade. Instead, she can become Gardevoir, the other evolution. You want to make sure you catch a male version on your travels before you go any further.
Once you've done that, the next step is to give him the Dawn Stone. The Stone of Dawn is one of the many evolutionary items you can acquire during your travels in Pokémon Brilliant Diamond and Brilliant Pearl. You can find the Stone of Dawn in a number of ways, but the simplest method we've found for locating it is to walk around with a Pokémon that has the Collect ability. They have a random chance to hold an object while you walk, and the Dawn Stone can be that object.
When you receive the item, give it to your male Kirlia and wait for it to evolve as it levels up. However, if she doesn't hold the Dawn Stone when she reaches level 30, she will become a Gardevoir.