Alphonse Elric is the younger brother (because his birthday is unknown, it is believed that he is around 2 years younger than Edward Elric), but is often thought to be older than Ed because his soul is bound to a suit of armor much bigger than Ed. Also, because of the armor, most people who meet Alphonse automatically assume that he is the Fullmetal Alchemist. When Ed and Al tried to revive their mother, Al's body was taken to the Gate and Ed had to save him by giving up his arm as the equivalent exchange to bind his brother's soul to a nearby suit of armor. Because of this, Al is unable to feel, smell, or taste anything anymore (he cannot eat or sleep, but can go into a 'hibernation' state where his eyes fade out), but he can apparently see, hear, and speak. Al, like his brother, is an alchemist. Before burning down their home, Ed painted the symbol of a flamel cross, which is a winged snake on a cross, on Al's armor. The symbol was inherited from their teacher Izumi Curtis and from Izumi's teacher Dante.
Al, with his calm and optimistic personality, usually acts as a foil for the fiery, excitable, easily discouraged Ed. But during his fight with Barry the Chopper at Laboratory 5, Al was troubled by the fact that he could possibly be a fabrication of Ed's and may not have truly ever existed, since he was unable to remember some things from when he was human and thought Edward was keeping something from him. As a result, Al left to find the truth for himself. But by helping Scar deal with mercenaries and Barry, Al comes to realize that he did in fact exist, and rejoined his brother in their search for the Philosopher's Stone.
After Kimblee uses alchemy to transform Al's armor into explosive material, Scar transferred all of his incomplete Philosopher's Stone into Al to save his life. As a result, Al became the Philosopher's Stone. Shortly after, Alphonse learns his body became lighter and his blood-seal could endure contact with water. He also gained the ability to perform alchemy without a circle. He only uses alchemy once after he becomes the stone because any alchemy he or anyone near him uses activates the stone and destroys a part of Al's body. Because he houses the Stone in his body, he becomes the primary target of Dante's attempt to cheat death, but not before he becomes a pawn in Shou Tucker's plot to resurrect Nina. Once captured by the homunculi for Dante, Al was to be devoured by Gluttony to complete the Stone inside Gluttony's body. But when he saw his brother killed by Envy, Al uses the stone's power to restore Edward's body (including his missing limbs) and re-bind his soul to it. This destroys his own body as he used up the whole of the Philosopher's Stone in the transmutations.
Ed, who could not accept his little brother sacrificing himself for him, resurrected Al by using himself as the equivalent exchange in a transmutation and fully expecting to die. Al came back in his original 10 year-old body with no memory of his life since the night they tried to revive their mother. He became Izumi's student again to study alchemy so that he could be reunited with Ed again someday. During that, Al became as childish as his older brother, donning attire similar to Ed's, equipped with gloves that have transmutation circles on both of them so he can merely clap his hands together to perform alchemy. However, Alphonse's alchemy is unique as he can now transmute a piece of his soul into inanimate objects (thus controlling them). After stopping an attack by the Thule Society on Liore, he meets Wrath and together they open a physical gateway between worlds through which the army of the Thule Society attacks Central. After helping to defeat the invasion, he follows Edward back to the Other World to help him close the physical gate. He then sets out with Edward to recover a nuclear bomb brought into our world.
Al is taller than his brother even in his human form, much to Ed's chagrin. Al has golden-blond hair and gold eyes like his brother and father in the manga, but caramel-blond hair and eyes that are an unusual sort of gray-gold-olive green color in the anime. It is believed that he is the more skilled martial artist of the brothers, as Ed states during his battle with Number 48 in the anime that "I've got nothing to worry about then. My 'companion' as you put it, I've fought him plenty of times and I've never beat him once." Ed and Al also mention something about them fighting over lots of stupid things and that Ed has never won before in anime episode 24.
It is also probable that Alphonse has a crush on Winry Rockbell. In a battle with Sloth and Wrath, Al was distracted by Envy, who pretending to be Winry, confessed her love for him. While the Edward-Winry relationship is more obvious, a more subtle interpretation is that Al and Winry have hidden romantic interests with one another. In an earlier episode, Al reminisces about an argument he and Edward had when they were younger, about which of them would get to marry Winry. The mere fact that this was still on Al's mind many years later indicates a greater depth of feelings.