^
You have to meet her in the Contest Hall first, and you must have beaten the fourth gym. She comes back when you chase the Galactic Grunt from Pastoria City to the Lake to the east (you can find him outside the Safari Zone gate). She then gives you the Secret Medicine thingy to make the Psyduck in Route 210 (west of Veilstone, north of Solaceon) to get out of the way. She then comes and makes you deliver something to the elder of Celestic. After that, she comes back to Hearthrome and you can challenge her.