On the streets of Midgar, he's a ghost. The crowds part to let him pass, compelled to move by some unseen, unexplainable force. Or maybe just the errant cold breeze—
But no, that's not right. He doesn't want to think of himself as nothing but a change in the temperature, even in the state he is, and has been, for what's grown to feel like an eternity. Maybe it's selfish of him, but he thinks that he worked much too hard in life to end up a simple, wandering spook. Even if that's what keeps him on the even keel, these days: watching the world. And watching him. The changes in Cloud are startling (he used to be that little recruit and he used to be so scared), but Zack won't let that deter him any.
Sometimes he even moves past the idle work of a spectator and lends a hand, extending himself as far out of the afterlife as he can; using what was once, so briefly, an infamous will in order to push those boundaries, break down the fluid walls. As hard as he tries, though, the little things are still all he can manage. He'll guide a hand toward the right decision (or the one he likes the best for Cloud, selfish again, yes he knows), or push him onto the trail of a misplaced item (the keys, for example, he's always losing those someplace, lucky he hasn't lost his job yet). Just the little things, usually.
Once he managed to write a note, though, a short, scratchy excuse for a message, but he never got to see it through. The second the words were out, he lost that precious control over his own identity, his own id in the bustling throughway of the Lifestream, and had to wait that precious moment out again when his energy could recollect itself into something passable. The message was useless anyway (love you Spike forgive yourself, please how are you), just a fragment. And he still doesn't know what became of the paper when he left it near the phone on Cloud's jumbled desk.
And maybe it doesn't matter, if that's right. He can still follow, he can still lead, and he can always be there when he knows there's a reason to be. He'll stay that warm, invisible touch, that sudden, reasonless sensation that comes and goes, and that could be enough to get by on. When it leaves him lonely, there's always Aeris, her smiles, her voice, laughing eyes…
But when he gets down to the heart of the matter, she isn't what he wants. He'll chase her to Hell and back if he has to, he'll keep her safe, he'll be comforting and careful, in this shared afterlife, but in the end, she still isn't what he wants. She's what he needs, a part of it, and he knows that she's worlds more reliable than anyone else he's ever chased. And still it's not the same.
On the streets of Midgar, being First Class anything doesn't seem to matter anymore. He's a specter, a useless spook; he can't get out of the city. He can't let his parents know that he never went MIA, that he would've written home, he really would have, if he'd only had a few more days in which to remember. He can't stop the monotony, but he can always take these walks, watch Midgar's little worlds spin and fall, and wait for Cloud to come back again. Because even in death, he'll always remain exactly the same, the diligent soldier in him will never change.