From Final Fantasy Wiki

Nanaa Mihgo is a Mithra Thief who resides in Windurst Woods. She is constantly pursued by the Star Onion Brigade (S.O.B.s) as she continues her plans to steal treasure. If the adventurer is doing missions for Windurst, he must travel to Nanaa Mihgo's secret hideout beneath the Horutoto Ruins to reclaim the stolen Book of the Gods. Upon getting there, Nanaa Mihgo and her two sidekicks threaten to rob the adventurer, but he is saved when Ajido-Marujido appears and rescues them all from the feral Cardians known as the "Aces". Nanaa Mihgo gives fame in Norg, which is required to unlock Leviathan, do Ninja scroll quests, etc. Nanaa Mihgo is also the central NPC in the Thief Artifact Armor quest as she attempts to recover the treasure of a legendary Thief.