Oh, holly chocobo... Some people are pushing too hard. They throw away something obvious and trying to find little details to pop up clues out of nothing. Some comments say that the credit song "Hollow" is about Zack, Tifa, even Sephiroth (this was a joke, right?)... This is really pointless.
Please, just listen the lyrics.
In the beginning of the game, Cloud is lonely and lost for a long time. He didn't feel like he belonged to Avalance, too. The person that really pulls him into action is Aerith. After they met, he changed from a character who worked as mercenary for Avalance, to a person who trying to protect other people and planet voluntarily. So, yeah, "her smiling face guide his way". Not Zack's smiling face, Zack was planning a future for them as mercenaries, he didn't guide Cloud to a sacred way or someting, he didn't give a real purpose to Cloud. Not Tifa's smiling face, because she was not smiling (no, I'm not kidding. Or... maybe a little bit but just wait), she was trying to convince him, trying to remind him of his past and Tifa was sad, angry. Moreover, it cannot be said that her efforts have worked well in the beginning. These two characters might have saved and supported Cloud, but these are completely different things from "guide his way".
Since Aerith is a healer, I don't even want to touch on that "heal every wound" part. But if we think of it as spiritual wounds, someone who doesn't see the wounds cannot heal them. And Aerith is the only one who can see. Anyone who knows him can recognize the profound change in Cloud's character, but the only one who can understand reason of this is Aerith, thanks to her abilities.
Now, we came to the real important part. The person that Cloud calls out is lost, gone, and Cloud says, "If I had realized, would you still be with me?" (actually he says, "would you be in my embrace?" and for me, this is not a brotherly expression, anyway). This is extremely clear. This person is dead, and the Cloud blames himself. He says "I was blind to everything, I was a fool, I didn't really care anything. If I could act differently, could I change what happened?" Oh, come on. How can Cloud be responsible for Zack's death? And Tifa isn't even dead or lost, she is always there. Be reasonable!
We all know that there is a tragedy behind Aerith's smile. Apart from that, Aerith is one of the best knowers of the danger that the planet is facing. "Dark mysteries that lurking beneath her smile" is not only her tragedy, but also probable dark future of the planet. But while Aerith describes what might happen to the planet, the danger in the future; what Cloud really cared about was his own mess. He was empty, he was selfish, and he was living with a void that he had to fill in. And remember, this gap made him make a mistake that would cost Aerith's life. That's why the regret in Cloud is growing. So, Cloud wishes another chance, this time he will see the facts that upset Aerith (darkness, or Sephiroth, or Jenova, whatever you say), and he will do something, do anything, and never let her go.
And this is the most touching part: Cloud wishes another chance but he knows it's too late. Aerith is already gone. And there is nothing he can do against it, he has to "go on hollowing and hollow". And we saw that he was doing this for two years in AC.
Soooo, there is no doubt. The person called in this song is definitely Aerith. This is a lament from Cloud to Aerith.
