It certainly seems like either Zeira is just in Mavis's imagination or she is some kind of ghost. The fact that Mavis ended up like that herself in the future would lend some credence to the belief that Zeira is there in some sense. Although she isn't there in the present day so...maybe she is just imaginary?
At the very least this chapter made clear that Zeira is not physically there. Whether she has substance or not is a question for another day.
Props to Yuri I guess for managing to keep a reasonably calm expression there. Though does make me wonder about his relationship with Mavis in the future and what will happen to him (you know aside from being the father of the third master of the guild
).