How to Unlock Meryleep as a Master
Getting Meryleep to apprentice you is a real pain in the butt! First and foremost you won't be able to start this side quest until Ryu is an adult and you recruit Peco to your party again. Once you've done this, head over to the ? in Eastern Wyndia and use Ryu to cut through the bushes to find the pond from earlier in the game. Use Peco to kick the rock into the pond and Meryleep will come out to yell at you.
She tells you that if you get her the Flower Jewel she'll reward you, allowing you to apprentice under her. This Jewel is located in Central Wyndia, there's a ? spot to the east of Maekyss Gorge/the town with the Arena. You have to travel around a mountain and you'll find it at a dead end.
This area is really small, there's a single tent here and a tree. If you look atop the tree you'll notice a small black figure up there, take a closer look and you'll see that it's a crow! With Peco in your party, approach the tree and hit Triangle; This will cause the crow to fly away and drop the Flower Jewel in the process.
Bring the Flower Jewel back to Meryleep and you'll finish this brief side quest/be able to recruit her as a Master.