Alfons Heidrich is Alphonse Elric's look-alike on the other side of the gate. He met Ed in Munich and together they study rockets. They live together in an apartment.
Sadly, Heidrich has a disease that makes him cough up blood, and he only has a short time to live. It's never said precisely what the illness is, but tuberculosis would be a good guess. Heidrich's dream is to have his rockets fly to other worlds before he dies.
Edward usually tells Heidrich about his adventures as an alchemist while he was in Amestris, but Heidrich doesn't believe him at all. He believes they're just fantasies made up in Edward's mind.
Although he might put his rockets before anything else, Heidrich has a big heart that cares for Edward.