Alex Louis Armstrong, known as the Strong Arm Alchemist, is a State Alchemist and a major in the Amestrian State Military. He was born into the esteemed Armstrong family, a lineage of elite alchemists who have held the highest military and political positions in Amestris for generations. He inherited his family's signature alchemical techniques which combine physical strength with alchemy, as well as a deep sense of honor and responsibility. Despite his towering, muscular physique and intimidating appearance, Armstrong is known for his friendly, overly affectionate, and deeply caring nature. He is highly emotional, often bursting into tears of joy, sorrow, or compassion. His motivations are rooted in a strong moral compass and a desire to protect his comrades and innocent people. He hates violence and always seeks to resolve conflicts peacefully, which sometimes puts him at odds with the harsh realities of his world.

In the story, Armstrong was one of the few military officers who objected to the slaughter of civilians during the Ishvalan War, for which he was sent home in disgrace, highlighting his empathy and integrity. In the present day, he works at Central Command alongside Maes Hughes and later joins Colonel Roy Mustang's coalition to oppose the corrupt central government on the Promised Day. He frequently assists the Elric brothers, Edward and Alphonse, in their investigations and battles, showing unwavering loyalty and commitment to justice. His key relationships include a complex dynamic with his sister, Olivier Mira Armstrong, who commands Fort Briggs; their interactions are marked by both humor and mutual respect despite their contrasting personalities. He shares a deep sense of camaraderie and mutual respect with Roy Mustang and the Elric brothers. Over the course of the series, Armstrong undergoes significant development, particularly as he confronts the moral ambiguities of war and the consequences of his past inaction, evolving into a more nuanced and resolute individual. His greatest strengths are his loyalty, compassion, and willingness to risk his life for others. His flaws include a tendency to be overly emotional and sometimes inadvertently causing harm due to his immense strength. His notable ability is a unique style of combat alchemy passed down the Armstrong line for generations, which blends alchemical transmutation with powerful physical strikes, allowing him to reshape the environment and fight effectively.