Rhyanna asked Ash and his friends to help find her brother after he ran away with his newly caught Murkrow. He was angry with her after she kept defeating him in battle over and over again. Her grandfather told her about the Dusk Stone; thus, she wanted to find it so that she could evolve her Misdreavus into Mismagius, which could use incantations to make Mitchell happy.
With Brock's help, she found the Dusk Stone. Unfortunately, Team Rocket intervened, captured her Misdreavus, and tried to steal the Dusk Stone as well. However, she would not let go of it and put herself in danger. She was eventually saved by Mitchell's Murkrow, who arrived just in time to help. Seeing that Mitchell was feeling much better, Rhyanna realized that she would no longer need to evolve Misdreavus. She and her brother decided to battle for the Dusk Stone, awarding it to the winner of the best of ten battles.
Misdreavus is Rhyanna's only known Pokémon, and using it Rhyanna was able to repeatedly defeat Mitchell's Murkrow. However when Mitchell ran away to train his Murkrow which started to play pranks on travelers, Rhyanna sought for a Dusk Stone so she could evolve Misdreavus into Mismagius and use its ability to create incantations to cheer up her brother. Although Misdreavus was able to quickly find a Dusk Stone, Team Rocket attempted to take both the Dusk Stone and Misdreavus, only for Mitchell to help save her with Murkrow. Afterwards, after seeing that she no longer had to evolve Misdreavus, both she and Mitchell agreed to battle for the Dusk Stone.
Rhyanna's Pokédex as seen in this episode appears to represent the Ice Blue edition of the Nintendo DS Lite that was released in 2008 (similar to how the Pokédex typically represents the latest released Nintendo handheld system).
From チルチル Tyltyl, a character in the Belgian opera L'Oiseau bleu. The sibling of this character is named ミチル Mytyl.