When Zack breaks Cloud out of the mako tank and realises just how badly the poison has taken root, his first instinct is to kiss him: to kneel over his lifeless form and suck the mako right out, until his friend’s lips glow with life again.
He’s groggy and slow and the extra mako that he draws from Cloud’s mouth is almost enough to send him back into unconsciousness, but he wills himself to go on, for Cloud’s sake.
It has no effect: Cloud remains unresponsive, a dead weight where once there was a vibrant young man. Later, when Zack has regained more of his senses, it occurs to him that getting Cloud out of his mako-drenched clothes might be more productive.
But he tries the kiss again after that anyway; it seems the most immediate way that he might help, and there is something comforting about pressing his lips to Cloud’s that reminds him of home, of two country kids off on an adventure that has grown bigger and more solemn than they ever anticipated. Talking to Cloud doesn’t seem to be getting him anywhere, but kissing him – connecting the two of them in a way that is so much more intimate – perhaps that can appeal to something in Cloud’s subconscious, call out to him deeply enough to break through.
It becomes a ritual. A kiss after they escape Nibelheim, with the Shinra guards taken care of. A kiss in Gongaga, after Cissnei has let them go, clearly trying to disguise her concern. A kiss when Zack returns from under Banora, when the story of Genesis has come to a close: he has never before felt so connected to Cloud. They are alone now, and all they have – all they have ever really had – is each other.
In the truck that will take them to Midgar, Zack kisses Cloud more fervently, more urgently. He can no longer pretend its only purpose is to draw out the mako: now it gives him comfort, helps him remember the friend he once knew, so full of life, so genuine. He wishes he could have kissed Cloud back then, five years ago; he wishes Cloud could have been awake to see in the strength of Zack’s kisses how much Zack cares for him, how willing Zack is to protect him over all else.
And then, when it all goes so awfully wrong, Cloud finally comes to, and Zack uses the last of his strength to entrust the Buster Sword to him; and the final sight he can process is Cloud lowering his face towards Zack’s with muddled but determined eyes, and the final touch he feels is Cloud’s lips against his own.