He is most alone in crowds. He's never had his brother's bright hair or eyes; his face is unremarkable, forgettable. When the Fullmetal Alchemist vanished, no one questioned his missing younger brother. Without Ed, he may as well not exist.
And maybe that's why he wears the coat. The coat is not his brother, but it's the closest Al still has. This was his brother's coat, and the thought makes him hold on tighter. He doesn't remember it from Before, in any of his remaining memories; Winry and Granny said they didn't remember either brother buying it. Ed must have made it, the same way he created Al: two hands held together as if in prayer.
The fabric must have come from their house, before they burned it. He tries to remember the color in their room, in any room, but he can't; he tells himself that it was a long time ago, but it really wasn't, for him, it was less than a year ago, for him, and this lapse in memory says more about him than he'd like to hear.
His brother made this coat with the same hand that bound Al's soul. He likes to think that their souls are bound together still, even all these years later, through this red coat and through their shared blood.
Teacher told Al to stay away from soul alchemy, told him it was dangerous, but he can't see any danger in the alchemy that saved his life twice over – his brother's alchemy – and sometimes, sometimes he wonders if it is because it reminds her of Ed.
Mother had loved when he and Ed performed alchemy because it reminded her of Father, but Winry and Granny and Teacher and everyone else – they never want to talk about Ed, especially to Al. They don't want to be reminded of him, they don't want to even think about him, because his name reminds them that they have given up.
Al hasn't given up. He uses his brother's name as something to give him strength, like a prayer, or a mantra, or as if he repeats it often enough, it will guide his brother home like a light in the window that never goes out.
His brother, who is the oldest son, the loudest son, the memorable son – Al knows they love him more, but he doesn't blame them, can't blame them because he loves Ed more too. Al has lived in his brother's shadow his entire life, and he has never minded. Even now that Ed is gone, he is still a shadow. People see his brother or they don't see Al at all; without Ed, he may as well not exist.
But Al is not his brother's shadow. They share their blood, this coat, even their souls are connected, but Al is not his brother, could never be his brother, and no matter how hard he tries to be him, it will not bring Ed back, he knows.
And he knows that his brother is still alive. After all they have struggled and all they have accomplished, he has to be. It wouldn't be equivalent otherwise. This was meant to be. They deserve a happy ending, even if they have to fight for it for the rest of their lives.
His brother is still alive – Al would know if he was not. He would be able to feel it, he's sure. He would. He would. He would know. He has to be right.
He has nothing left to live for if he's wrong.
And so he is searching, because an ideal is worthless if not put into action. His brother is alive, and Al will find him. And just as Al is searching, Ed must be searching as well. He would never leave Al if he could help it. He had never forgiven their father, and he would never forgive himself. He hadn't left by choice; it has to be that something forcibly keeps him from returning home, something beyond his control. It has to be. It has to.
He isn't dead, only lost, and all Al has to do is find him.
He wears the burden of their sins on his back in the color his brother came to own. Red is the color of all he has lost, but he wraps it around his shoulders to remind himself why he searches. He won't allow himself to forget.