User blog comment:Omega natsu2/Fairy Tail Zero: Night of Embarking, Review./@comment-24102956-20140914172712/@comment-25111584-20140915092615

Expanding on B214's theory, the Jade might have some sentience of its own and might actually have hidden itself inside Mavis during the Blue Skull attack and then decided to stay there, maybe even protecting her from predators on the island. As Mavis seems quite ... impulsive ... it might manifest itself as Zeira to not only act as a voice of reason, but also to keep her company.

... Though I still hope Zeira is just Mavis' imagination. >.>