Find 
Mog in the Narshe mines and bring him deeper to recruit Umaro.
Walkthrough
In World of Ruin Narshe, find Mog in the moogle caves. Defeat the frozen esper on the cliffs for Tritoch, then explore the cave opened beyond it. Examine the bone carving for Terrato and defeat the yeti; Mog can persuade Umaro to join.
Return to the Solitary Island beach after obtaining the Falcon and collect Palidor. On Triangle Island, encounter Zone Eater and allow it to engulf the party. This is the entrance to an optional interior, not a normal battle victory.
Inside, cross the narrow bridges and timed hazards carefully. Speak to 
Gogo at the far end to recruit the mimic. Both Gogo and Umaro are optional, but they increase your choices when assigning final-dungeon teams.
Checkpoint
Enter Narshe with room in the party for Mog. After recruiting him in the moogle room, search the spot behind him for the Moogle Charm, which prevents random encounters when Mog equips it. On the cliffs, the 
Ice Dragon is optional; defeating Tritoch opens the way to the yeti cave. Bring Mog before confronting Umaro.