She looks exactly like Dawn, only differing in appearance by her hairstyle and clothing. Wishing to escape from her royal duties and compete in a Pokémon Contest after all her hard training with Togekiss, the princess swapped identities with Dawn.
Using her Togekiss, Salvia won against Jessilina in the final round, almost being defeated, 'Dawn' successfully earned the Katakuri Town Ribbon. Out of kindness, she gave the Ribbon she won to her opponent, rather than keeping it for herself. Also, to allow her Togekiss to compete in more Contests, she gave it to Dawn.
She appeared in a flashback in DP172 and briefly appeared in DP177 watching the Sinnoh Grand Festival Final Round on television.
[credit to bulbapedia]