User blog comment:Bato of the Heavy Iron Rock Hammer/Zeira dead or alive?/@comment-24066070-20140915074855

Mavis is actually Roubaul's descendant who inherited his powers of "being a real, tangible ghost while being deceased", as well as creating illusions which are so strong they are experienced as real (this is how she manifests Zeira)

Lumen Histoire will be a second Nirvana which turns all the remaining Demons into good people and use it's canon to blast Zeref and Acnologia to smithereens.

There, now you know who/what Zeira is and also how the entire plot of the series will unfold.