Alphonse "Al" Elric (アルフォンス・エルリック, Arufonsu Erurikku?), is a fictional character in the Fullmetal Alchemist universe created by Hiromu Arakawa and developed into a media franchise, consisting of a series of manga, anime, soundtracks, OVAs, movies, video games, and even collectibles. He is a human child who lost his body during an alchemic experiment to bring his deceased mother back to life, and had his soul transmuted into a suit of armor by his brother, Edward Elric.

Al's blood seal lies just below the base of the neck at the back of the suit, on the inside. It is drawn with Ed's blood. According to Edward, he needs to be careful whenever he reconstructs Alphonse's body because he must keep the blood seal intact during the transmutation. If the blood seal is removed, Al's soul will be lost.

Source: Wikipedia