(This is for the fanfiction "Crossover Together". Info may change from canon information)
In his gunmetal-colored steel armor body, Alphonse stands out as one of the tallest members of Team Harmony, inching out such stalwart man-mountains as Alex Armstrong while absolutely dwarfing his older brother. As such, he is mistaken for being the famous "Fullmetal Alchemist" in place of his brother - the rightful bearer of the title. The helmet that comprises Al's face is adorned with a large, conical horn in the middle of the forehead, has a pair of parallel, horizontal vents beneath each half-moon eye hole and a fang motif just above the protruding chin. The top of the helmet also sports a ribbon of white hair that hangs down to the small of the suit's back.
Alphonse's appearance before the accident was rather similar to his brother's. He was taller than Edward even then, despite being a year younger, and wore his golden-blond hair short and neat with a parting on the left side. His eyes - gold like Ed's - are rounder than his brother's and set in a broader face.
In many ways, Alphonse's calm and passive disposition exists as a foil to his older brother's more abrasive personality. Al is kindhearted, notably compassionate and patient almost to a fault, acting often out of protective selflessness for his loved ones, new friends and sometimes even toward his enemies (Envy as a notable example). Having had his psychological growth stunted at the age of ten due to loss of his physical body, Alphonse retains a childlike purity in the face of all his encountered horrors; however, despite his age, he has always displayed a significantly higher level of maturity than Edward.
At a young age, he and his family were one of those that boarded the first Hybrid Train. This train ride resulted in him getting killed along with everyone else, only to be revived by Undertaker after Alex Armstrong convinced him to do so. However, Alphonse's body was so deeply damaged that his soul had to be placed within a suit of armor, his memories altered to believing that his brother put him in that body to save his life.
He traveled with his brother for the years to follow, helping various towns as a supporter to the "Fullmetal Alchemist", up to the point when he came across Olivier Armstrong (and nearly killed for the alchemy). Since then, as a promise to Olivier, he had to become more secretive, and he himself had to remain isolated in their travels. It is revealed he became one of Olivier's students afterwards, though unclear as to how.
As a full-fledged Amestris soldier, he was with his brother on their way to Central when he first came across Blair (in her cat form), and decided to take her in for a bit. He was later found out by Ed though, and he tried to keep Blair to himself as he didn't trust Sebastian Michaelis when he arrived to get her. Due to the convenience of where they were going, Sebastian helped Alphonse, Edward, and Blair catch up to the train (leaving him terrified on arrival).
After arriving to Central, and when the Pluton incident occurred, Alphonse went out into the field to try and locate some survivors of the attack. He and Blair both end up finding Envy all alone, and feeling sorry for him, ultimately decided to bring him back. He brought him in secret outside of Central, only was exposed at the tail end of it once they arrived. He managed to convince everyone to let him stay and heal.
As apart of the protection agreement, he went with his brother to Tokyo, and became a student at Orora High School. He's mainly support towards Ed throughout the season, and it was him who saved Karma's like from Karasu, who would've killed him otherwise.
Just before the groups went off to hunt their Demons, Alphonse was found by Barry; a fellow victim of transmutation. His encounter with him got Alphonse to start to question his being, as Barry brought up the idea that his entire history could've been altered to suite his "brother" best. He tried to ask Ed about it, but didn't get enough time to get a proper answer or explanation.
He left with Ciel Phantomhive and Sebastian Michaelis on board the Campania, though he had to wait within storage in order to get himself on. He had to keep his body hidden, and only went with the others just as a head being carried around. He later saw the presentation in the Aurora Society by Aleistor Chamber, but unlike practically everyone else there, he didn't buy the act and believed he simply restarted her physical functions, instead of actually reviving the person. Once alone with Ciel, he asked him if he would alter someone's memories if he could transmutate someone, being told that he would, but would want his true brother over a alteration. His doubts were dashed after the conversation, but he was unable to help at all when it came to the full confrontation, and return of Sombra.
He found about his true past once he returned to Tokyo, though it was unclear how he really felt about the situation.
Alphonse became one of those captured by the Slicer Brothers, and taken to Abaddon. Unlike the other prisoners, he was confiscated like the other weapons, and left in isolation until the others came and rescued him.