Rui is a main character from Pokémon Colosseum. Many have tried to capture her because of her ability to see the unique aura surrounding Shadow Pokémon. After Wes saved her from a member of Team Snagem, she joined him in his quest to save the Shadow Pokémon.
In the English version of the game, Rui's skirt and shirt were lengthened.
Rui's ability to see aura more closely resembles the Aura reading ability that Aura Guardians use, which may explain how she is able to see Aura in the first place.