Ashleigh Crichton is a central figure in the narrative of Fullmetal Alchemist: The Sacred Star of Milos. He is the son of alchemists from the region of Milos and the older brother of Julia Crichton. His family was driven from their home by hostile Milosians and subsequently placed under the protection of a Cretan security chief named Atlas. As a young man, Ashleigh became deeply interested in his parents' research, which concerned the secrets of Milos and the creation of the Sanguine Star, known elsewhere as the Philosopher's Stone. His life was shattered when he discovered the scene of his parents' murder and was attacked by their protector, Atlas. The attacker stripped the skin from Ashleigh's face and removed a patch of skin from under his arm that bore a tattooed map of the ruins of Milos. Left to die from blood loss, Ashleigh survived by swallowing a Sanguine Star that his parents had hidden nearby. The Star's power kept him alive but was unable to heal the wounds to his face.

Driven by a deep-seated need for revenge against Atlas and a profound bitterness toward the people of Milos for their mistreatment of his family, Ashleigh infiltrated the Cretan military under the alias Herschel. To hide his disfigured face, he donned a white mask and rose to the rank of Colonel. His personality is shaped by trauma and a thirst for vengeance. He is calculating and patient, having spent years orchestrating his revenge and using his military authority to keep his younger sister, Julia, under secret surveillance by chimera guards, waiting for the day Atlas would come for her half of the map. However, beneath his cold exterior lies a deep-seated rage and a desire to create a new world according to his own sense of order, a goal that ultimately corrupts him. When he finally confronts and kills Atlas, Ashleigh chooses to continue the man's destructive rampage, seeking to annihilate the Milosian people as retribution for their past cruelty. Edward Elric confronts him on this point, accusing him of being unwilling to accept reality.

In the story, Ashleigh begins as an unseen guardian, using his position as Colonel Herschel to pursue Atlas. His true identity is revealed late in the film when he personally kills Atlas. At this point, he assumes the role of the primary antagonist, revealing his plan to use the power of the Sanguine Star to destroy Milos. His motivations shift from personal revenge against a single man to a genocidal crusade against an entire populace, believing this act will restore order to the world. His most significant relationship is with his sister, Julia. He spent years protecting her from afar but was unable to be with her due to his injuries and quest for revenge. Their bond is symbolized by a pair of earrings they share. While he cares for her, his hatred for Milos ultimately overrides his love for her, leading to a violent confrontation. Julia, however, refuses to give up on him, and in their final battle, she uses the power of her own Sanguine Star to fatally wound him while trying to stop his rampage.

Ashleigh undergoes a significant, albeit tragic, development. He starts as a victim seeking justice but transforms into a perpetrator of the same kind of violence he suffered. His all-consuming hatred twists his original goal of avenging his parents into a plan to commit mass murder. In his final moments, as he lies dying from his sister's attack, Julia uses the remainder of her power to heal him. She restores his face and strengthens the weakened connection between his body and soul, an act that costs her a leg. Ashleigh awakens in a hospital, his face restored. Instead of reuniting with his sister, he leaves quietly, taking one of the pair of earrings as a silent token of his continued care. He then dons his mask and Cretan uniform once more, returning to the military and his alias, Herschel, indicating that while his body is healed, he remains a person shrouded in bitterness and secrecy. His notable abilities stem from his alchemical knowledge and the power of the Sanguine Star he swallowed. This power allows him to perform powerful transmutations and sustain his life despite his grievous injuries. He is also a capable military leader and fighter, commanding chimera soldiers and engaging in alchemical combat.