There were a lot of things that people did not know about Edward Elric.
For instance, only Alphonse knew that the day after the incident with Barry the Chopper, Ed had bought a gun a learned how to use it, and use it well. He was an excellent shot: good enough to rival even Riza Hawkeye, although it had taken a lot of practice.
Only Alphonse knew that after Brigadier General Hughes died, Ed had learnt how to use the throwing knives the man had been so accurate with. Again, it took a long time, but Ed became proficient in using the damn things, despite a few rather painful mishaps.
Only Alphonse knew that his brother had spent long hours in the kitchen, learning how to cook after their mother died. This proved to be a relatively easy task: Alchemy began in the kitchen, or so it is said, and Ed's natural affinity for alchemy seemed to extend to cooking.
Only Alphonse knew that Ed was an excellent dancer: it was good exercise, when they couldn't find enough space to spar in, and it worked just as many muscles.
Only Alphonse knew that Ed was extremely violently opposed to any kind of chimera creation, because of Nina Tucker. Only Alphonse knew that, despite Ed's brash personality, his brother was a caring and intellectual young man, albeit hot-tempered. Only Alphonse.
Only Alphonse truly understood how strong his brother was, and how weak. Only Alphonse could truly understand how strong-willed his brother was.
How determined do you have to be to still be lucid enough to draw an array after losing a leg? Mustang, for all his clever tricks, could never truly understand how strong Ed really was. Did Mustang realise, Al wondered, how much pain Ed had been through to have his automail installed, and yet barely even made a sound?
People often saw the automail, but rarely did they realise what it actually meant. It meant that the limbs were gone. Oh, sure, people realised, knew, on an intellectual level, that automail meant no limb. But Alphonse wasn't sure they truly understood that until they saw it removed.
People knew that automail was painful, but Al wondered if they knew how much pain Ed was in, a constant, dull ache from having cold, hard, unforgiving metal bolted permanently to his body. How much energy it took to sustain, how heavy it was. How problematic it could be. Did they realise that his brother had to clean it every day, oil it, keep it free from dust and dirt? Did they realise that the slightest problem could render Ed completely helpless, unable to walk or use his right hand?
Only Alphonse knew how much his brother was willing to go through for what he considered a good cause. They knew that Ed was dedicated- but only Alphonse had actually seen how much Ed really cared.
Ed, bleeding heavily from his left leg, suffering still from the pain of losing their mother, exhausted by the transmutation and absolutely terrified- had still been able to draw out the transmutation circles needed to literally drag Alphonse's soul back from the very gates of death.
They said the dead were dead, but they were wrong- to a certain extent. People could be brought back from the dead, and Al was proof of that, even if he wasn't exactly alive in a very conventional manner. He had died, back then. Ed had brought him back. Ed, who had only been eleven years old. Only Alphonse.
But only Alphonse knew how easily his brother could be hurt. How simple it was to terrify his brother, scare him, render him totally helpless.
Only Alphonse had seen his brother break down after Nina's death. Only Alphonse knew how much their failed attempt at human transmutation haunted his brother. Only Alphonse really knew, really understood how easily his brother could break. He'd seen it at Lab Five, he'd seen it with Tucker, he'd seen it the day they attempted the forbidden.
Only Alphonse. Only Alphonse.
And only Alphonse would ever know.
After all his brother had done for him- well, keeping Ed's secrets was only equivalent exchange.