In his younger days.
He served Eria the Water Charmer as her familliar.
During this time they fought many battles together
He served her loyally because when he was younger, Eria had saved him from being eaten.
Eventually he grew up,
Into a formidable being.
One day, when fighting the forces of evil, Eria sacrificed her soul to cast a spell.
In an attempt to save his only friend, Gagagigo possessed her body to keep her alive.