There were many things that could not be described with turns of phrase, with a simple word that encompasses every feeling he could imagine into one neat little package, tied with a bow and handed to someone on their birthday.
Cloud would find himself watching Zack sometimes, the fluid way he moved and how he was constantly in motion even when he was asleep. Tiny twitches right above his left eye, his fingers tapping out a dance on the table when the food was slow to come. Zack would ask him what he was thinking and Cloud would shake his head, turning his eyes away. He felt like anything he was thinking was out there, written all over his face in bold script for the world to see.
Was it hero worship? Certainly, Cloud wouldn't mind to be a little more like his friend. Zack exuded confidence easily, his smile blinding and his manner a comfort to those around him. Even in a dangerous situation, Zack was quick to flip that switch from 'jovial' to 'dangerous' in an instant, his mind able to move five steps ahead of whatever fight he was facing.
He would watch Zack more often as time closed in on them, before the mission to Nibelheim. Zack would disappear in the afternoons when they were in Midgar, and when he returned there would be a bounce to his step, his smile a little more dreamy than Cloud could remember it being. In some ways it made him feel a little envious-of what, he couldn't say exactly. Part of it was how carefree Zack could be, and another part was how he never managed to make his friend look so happy with the world.
When his thoughts would turn in that direction, he would pause, and seriously what. Instead of thinking about it any further, he would go to the training room, working his body harder than ever before. It was his way of running, from thoughts that left him confused about himself and his friendship with Zack.
Usually, Zack would show up. He would chastise Cloud for tiring himself out, warning that without restraint he could seriously injure himself. Cloud would shrug in response and Zack would laugh, throwing an arm around the blonde's shoulders as he lead him out of the room, chattering about something that would have them both doubled over in laughter within minutes.
When someone volunteers to fight, for a person or an ideal, those friends made in the process can't simply be labeled friends. Closer than confidantes, a comradiere that isn't explained or given definition in any terms available. Cloud found that he was happier with it like this, with something a little more than a friendship not limited by words.