Colonel Roy Mustang (ロイ・マスタング, Roi Masutangu) is a fictional character from the Fullmetal Alchemist manga series and its adaptations created by Hiromu Arakawa. In the series, Mustang is a State Alchemist of Amestris' State Military, as well as the superior of the series' protagonist, Edward Elric. Mustang holds the title of "Flame Alchemist" (焔の錬金術師, Honō no Renkinjutsushi) for his ability to create fire with alchemy, and he ambitiously strives to become the next leader of Amestris. Despite his ambition, as the series continues, Mustang decides to overthrow the State Military after his best friend, Maes Hughes, is killed by the homunculi, who are controlling the Military.

Besides his appearances in the manga and the anime, Mustang has also been featured in other media for the series, such as Makoto Inoue's light novels, the original video animations and the Fullmetal Alchemist video games. Since his introduction in the manga series, Mustang has been well received by readers, appearing second in each popularity poll for the series. His character has also received praise in various outside media, with many of them focusing on his character and development in both manga and anime.

Roy Mustang is introduced in Fullmetal Alchemist as a 29-year-old State Alchemist working for Amestris' State Military and as Edward Elric's superior. Outwardly arrogant and playfully manipulative, Mustang is intelligent and adaptable. At the beginning of the series, he appears as a ruthless careerist and a womanizer. He is also eventually shown to be a rather paternal commander who greatly cares for the emotional and physical well-being of his men, which earns him the fierce loyalty of his subordinates. This paternal nature stems from his experience in the Ishbal Civil War, where he was forced to kill using his flame alchemy, despite having learned it to help people. Angered with how he was used in the war, Mustang resolved to become the country's next leader. An extremely powerful alchemist in his own right, Mustang wears gloves made from "ignition cloth" with transmutation circles that create sparks or flames when he rubs his fingers together. By adjusting the oxygen densities in the surrounding atmosphere through alchemy, he can create flames anywhere in the surrounding area at will and manipulate them as he desires with "pinpoint accuracy". While it is often commented that Mustang is unable to produce fire with his gloves wet, he can as long as he has some way to call up flame.

Following the death of his best friend, Lieutenant Colonel Maes Hughes, Mustang investigates the incident in secret despite the military closing the case after convicting Maria Ross. Trusting in Ross's innocence, Mustang fakes Ross's death and engineers her escape from Amestris. While breaking into a military laboratory, Mustang kills Lust, an immortal creature known as a homunculus, who was investigating his actions and was going to kill subordinates. Recovering from his wounds, Mustang learns that the Führer King Bradley is also a homunculus and tries to expose him to the top echelons of the military. This move costs Mustang and deprives him of his subordinates - Bradley places Hawkeye, Mustang's adjutant, under his command and authorizes the transfer of Mustang's remaining personnel to the far reaches of Amestris.

Mustang is later contacted by General Olivier Armstrong, who is intending to join forces with various troops to attack Central City, the capital of Amestris. He then meets up with his subordinates, and the four attack Central's military while Bradley is gone. Mustang later confronts the homunculus Envy after learning that he was Hughes' true murderer; fueled by rage, Mustang easily defeats Envy. As Mustang prepares to finish Envy off, he is convinced by Hawkeye, Edward, and Scar not to kill him and allow his own thirst for vengeance to consume him. Later, the homunculi attack Mustang, forcing him to use human transmutation to become the fifth human sacrifice needed for their leader. This results in Mustang losing his eyesight as part of the sacrifice, although he continues to fight with Hawkeye to help him direct his attacks. However, after the final fight is over, a former comrade, Tim Marcoh, offers to use a Philosopher's Stone to restore Mustang's eyesight on the condition that he promises that he will be a part of a movement to restore Ishbal. Mustang accepts and is put in charge of the East region as brigadier general.

Mustang's characterization and role in the first anime is similar to that of the manga until after Hughes' death, while his backstory is mostly altered from that of the manga. Prior to the show's start, the viewpoints of Ishbal's religion and distaste of alchemy lead to the friction between them, which escalated into a war when a military officer shot an Ishbalan child. After seven years of fighting, the state alchemists including Mustang were called in to end the war, during which, Mustang executed Winry Rockbell's parents who were doctors on orders by the military due to them helping anyone whether they were on their side or not, even Ishbalans dying in the war. This act ultimately caused Mustang great depression to the point he almost committed human transmutation until Hughes snapped him out of it and gave him newfound resolve to become the Führer of Amestris so he would never have to follow such unreasonable orders like that ever again. In the present, after Hughes' death, Mustang investigates his murder, but suddenly suspends it almost immediately after, much to the dismay of Sheska, who gives Mustang multiple books and information that Hughes was investigating, but Mustang has difficulty understanding them. Later, while hunting down Scar, Mustang and his squadron resolve to keep the peace between the military and the Ishbalan refugees. The Elrics themselves are eventually sent to conduct an uprising in Liore after learning that Scar plans on creating the Philosopher's Stone within said city by luring the military as sacrifices. Mustang attempts to get Führer King Bradley to hold off to prevent the military from going into Liore and thus, give them time to arrest Scar for his crimes, but is unsuccessful and the soldiers attacking Liore were eventually sacrificed, fully forging the Philosopher's Stone.

Forced by Bradley to pursue the Elrics after the Stone's creation, Mustang learns that Bradley is a homunculus and tries to expose him to the top brass of the military by revealing the truth about the Führer's secretary, Juliet Douglas. Mustang also finally comes to the true realization of all the corruption and institution plaguing the military which got Hughes killed in the first place. Edward and Mustang, despite their rocky relationship all this time around, finally repair it, coming to a greater understanding of each other before going their separate ways. Although Bradley intends for him to die in combat during the Northern Campaign, Mustang instead stays in Central, making his move to avenge Hughes. Mustang then moves on to the Führer's mansion and stages his fight. Mustang is only able to defeat Bradley when his son, Selim, unwittingly brings the skull of the man from which the Führer was created and weakens Bradley. As Mustang escapes the Führer's mansion, he is confronted by a maniacal Frank Archer, who shoots him unconscious, losing his left eye in the process. Hawkeye arrives in time to save Mustang's life and kills Archer, after which Mustang is forced to wear an eye patch while temporarily bedridden.

In Fullmetal Alchemist the Movie: Conqueror of Shamballa, Mustang resigns from both alchemy and his rank to become an ordinary enlisted man in a remote outpost. However, when Central is under attack by armies and airships from a parallel universe, Mustang steps up and takes command, using his alchemy to defend Central. Mustang and his comrade Alex Louis Armstrong are able to find a hot-air balloon to reach the airships, where he is reunited with the Elric brothers and helps them gain entry into the airship. Mustang and Alphonse are sad to watch Edward leave for good and destroy the gate on the parallel universe's side, knowing Edward will be trapped there permanently, but Mustang at the last second allows Alphonse to go so he can be with his brother. At the end of the movie, Al says that Mustang and his team are now responsible for destroying the Amestris side of the Gate.