Meryleep becomes available as a Master as soon as you receive the Faerie Tiara from Cadis at Lighthouse. You must also give her the Flower Jewel, which can be found in the mountain area just west of Maekyss Gorge bridge. You must have Peco in your party when you visit her.
Meryleep is the best option for raising a character's Agility due to her +2 stat boost. Additionally, next to Deis and possibly Ladon, Meryleep has access to some of the best Skills in the game.