Alphonse's soul is bonded to a suit of armor after a botched attempt by Edward and him to revive their mother had cost him his entire body in exchange.
The calmer of the two, Al is tasked with keeping his brother in check. Due to Edward's shorter stature, especially compared to his larger suit of armor, he is often mistakened as the older brother by others (much to Ed's chagrin) and for a robot by those outside the fanbase.
This tag applies to both his human form and suit of armor form.
