Instructions
Complete the game through the end of Disc 2. Amarant is the last character to join your party. Amarant will briefly, but not permanently, join your party after you beat him in the Madain Sari kitchen at the end of Disc 2.
Complete the Ispen's Castle quest on Disc 3. When you approach the castle, Amarant will leave your party and claim that he works best alone. Proceed without him through the castle and defeat the monster Taharka at the end of the castle.
Save Amarant in Ispen's Castle. After you leave the castle, you will notice that Amarant is not present. Return to the castle and climb down the ledge on the bottom of the screen. Zidane will engage in a discussion with Amarant. Afterward, Amarant automatically joins your party for good.