There were many things that Edward Elric didn't want the world to know, secrets to be protected at all cost. A mere handful of people knew that he'd attempted human transmutation, much less that he'd been successful the second time. Even fewer knew that he was still alive, and only two knew where he lived, but Al knew everything anyway.
But Al knew all the little things about his brother that nobody else did, the secrets that would not destroy them if they were revealed but were kept anyway. The trivial things that nobody cared about, but which Al had made a point of learning. For instance, Alphonse knew that there was a spot behind Ed's left ear which, when touched, caused him to shiver with pleasure, while worrying his earlobe with teeth and tongue resulted his entire body visibly relaxing. He knew that Ed loved having the arch of his right foot massaged, and knew the way Ed would purr with satisfaction whenever he did so. He knew Ed liked having his hair washed, the barely-there lines of tension in his face easing as the soap was rubbed carefully into his scalp. He knew exactly how Ed liked his coffee, and exactly how Ed liked to be woken up, and exactly how Ed liked to be kissed. He knew he was the only one who would ever know that running your tongue over Ed's hipbone caused him to curse with want, while nipping his collarbone ended in Ed writhing silently.
But he knew, too, how to calm his brother when Ed awoke from one of his nightmares, soaked with sweat and shivering. He knew where on Ed's shoulder to press in order to alleviate some of the ache from the automail, and that Ed's left leg hurt him in cold weather. He knew all of Ed's expressions and moods, and trusted himself to be able to read his brother better than Ed could understand himself. He knew how Ed's occasional distant or saddened expressions didn't relate to him, and had worked out exactly how to chase them away, though they always returned.
And he knew not to follow when Ed slipped away in the dead of night, unpeeling himself from the warmth of his brother to sit by the dying fire in the living room, alone with his thoughts and fears. But when Ed returned, Al would wrap an arm around him and nuzzle against Ed's neck, and Ed would smile—just for him.
Al wished he could chase the sadness away completely, but that was impossible. But sometimes, seeing Ed's expression whenever he said I love you, Alphonse thought he might be close.