I wanted to be by your side, Eiko... so I became Mog. Don't worry. I'll always watch over you, kupo.Mog, after going into Trance
Unlike the other moogles Eiko lives with in Madain Sari, Mog has a fur "collar" around her neck, and a golden pom-pom. Mog has pale pink fur and small bat wings. She is still young and is smaller than most other moogles. They share Ribbons as a sign of their friendship, which Eiko hopes they can wear together when they are older.
Mog is timid, and considered a coward by the other village moogles; as a result, she often runs away even when Eiko is in trouble. She accompanies Eiko wherever she goes, usually by shrinking herself to hide in Eiko's clothes. According to Eiko, she and Mog were born on the same day and have been thence inseparable. Due to her youth, Mog cannot communicate well with anyone except Eiko and the other moogles, speaking only in a language formed from permutations of kupo. Mog can be brave when she needs to, revealing a hidden side to her character.
Mog and Eiko attempt to steal food from Conde Petie and flee into the Conde Petie Mountain Path, but Eiko gets stuck on a tree branch. Zidane's party catches up with the pair, and Quina singles out Mog as a consumable. While Quina chases Mog back to Madain Sari, the remaining party members help Eiko down, and follow Mog back to the summoners' village where Mog is recovered, Quina having turned their attention to fish.
In Mount Gulug, when Eiko is at the mercy of Zorn and Thorn who attempt to extract her eidolons, it is revealed that Mog is the eidolon Madeen in disguise. Mog enters a Trance and urges Eiko to use her special ability, Terra Homing, which eliminates Zorn and Thorn. After the battle, Madeen tells her former charge that she will always be with her, before disappearing. Mog leaves Eiko her ribbon, which Eiko can equip as an add-on to learn the summon Madeen.
The writing on the Eidolon Wall suggests that Mog was born out of the dying summoners'/her parents' wish to continue to protect her:
Eiko, You are a treasure born to a dying village. We are dying, and there is little we can do for you. We shall return to the stars with one wish in mind. If legends are not born of eidolons, and eidolons but born out of legends, let us leave this place with one wish: If we can give birth to a legend, it shall create an eidolon that is friend and protector of humans.Message on the Eidolon Wall
Location: Memoria, Fat Chocobo in Chocobo's Paradise
The support ability Guardian Mog, is a unique ability for Eiko from a Ribbon or a Madain's Ring. With the ability equipped, Eiko is cleansed of status ailments she may have been afflicted with during battle, before the Experience and Ability Points are distributed to the party (except for the Knocked Out status). Even though Eiko can learn the ability early it does not function until the party receives the first Ribbon item.
The artwork for the Eiko summon's rank seven and eight phantom stones depicts Mog standing alongside Eiko. In addition, a moogle representing Mog appears during the animation for Eiko's Terra Homing summon ability.
Mog appears alongside Eiko (who is a summonable vision and optional playable character) in her 6-7★ sprites. Mog flies a little above Eiko's head, and is seen helping Eiko perform (or even copy) all of her actions.
Mog appears in Final Fantasy Trading Card Game as Water-elemental Backup cards.