I feel like it might have been a manifestation of his guilt but a ghost doesn't sound like too bad of an idea.

Maybe hallucination.

I think he’s “losing it.” He has so much guilt and is trying so hard to help everyone else, but hasn’t bothered to try to help himself. So she’s “there” to give him an audience for his grief. A psychiatrist would be better but much less interesting. This explanation for her presence makes it easier to swallow for me. I actually find it very interesting. If she is supposed to be a ghost, she’s on the wrong show. And to have her be a ghost takes a show about REAL issues and makes it UNREAL and much less compelling. Just my thoughts on it. ?

Its a manifestation of his guilt. Ghost!Hannah never really tells him anything new -- its just his way of coping and trying to sort out the mess he's in.

It's just flash backs he remembers when she was alive.

I really like the idea of Hannah being a ghost. Of her not being able to leave, or Rest In Peace, until Clay finishes his unfinished business. As he replied to Tony saying “You can’t love someone back to life”, you can try. And maybe he did.