Greil is the father of Ike and Mist in the game Fire Emblem: Path of Radiance as well as the founder and leader of the Greil Mercenaries, a mercenary company that operates within the nation of Crimea. At the start of the game, he considers his son Ike, on the cusp of elevating from trainee status to becoming a full-fledged mercenary, immature and with much yet to learn. He is content with accepting what others consider "small" jobs, such as driving bandits from local villages, but things change dramatically when word that the army of the neighboring nation of Daein has invaded Crimea. He sends Ike and other members of the company out to scout for intel, and when they return to the base with a young woman that claims to be Princess Elincia Ridell Crimea and are subsequently besieged by Daein soldiers, he chooses to do as the princess requests and protect her until they can reach the nation of Gallia and deliver Elincia safely to King Caineghis.
As the company flees Crimea, they stop to rest at an old keep. While encamped, Greil senses something off and leaves in secret for the forest in the middle of the night. Ike follows him and watches as Greil confronts the Black Knight; one of Daein's Four Riders, or highest-ranking generals. The Black Knight, a mysterious figure clad in ebony armor, refers to Greil as "General Gawain, Rider of Daein," and tosses Greil a sword, Ragnell. Greil refuses to wield the sword, instead choosing to fight using his own axe, but is mortally wounded in their duel. Hearing roars in the distance, the Black Knight escapes before he can finish Greil off. Ike tries to take his father back to camp, but Greil dies soon after, leaving the Greil Mercenaries in his son's hands.
It is later revealed that Greil's real name is in fact Gawain. He was one of the Four Riders of Daein dating back to the reign of the previous Daein king. His wife, Elena, was at the time a priestess of Daein's Palmeni Temple. When King Ashnard imprisoned a member of the heron laguz clan at the temple with the intent to use her to unlock Lehran's Medallion, Elena took pity on and befriended the heron princess. In return, the heron princess entrusted Elena with the medallion and also taught her the melody of a galdr called the Song of Release.
With Elena in possession of the medallion, Gawain renounced his post as a Rider of Daein and fled the country with his wife in order to keep it safe, out of Ashnard's grasp. The two eventually settled in Gallia, where Gawain changed his name to Greil. During their time in the beast laguz nation, Greil became a friend and ally of Caineghis, and his wife gave birth to both Ike and Mist. However, this peace is eventually shattered on a day that Greil makes the mistake of taking hold of the medallion himself. The chaotic energies of the god trapped inside it drive all that hold it insane, save for those such as Elena whose spirits are filled with balance. Under the medallion's influence, Greil engages in a senseless spree of violence and murder, and only returns to sanity when Elena allows herself to be impaled by his sword so that she can take the medallion from his hand.
Horrified by the death of his wife at his own hand, Greil injured the tendons in his sword hand so that he would no longer be able to wield such a weapon. Mist, who was discovered to have a spirit similarly balanced like her mother's, was allowed to keep the medallion, with Greil enforcing a strict rule that only she was allowed to hold it. Greil then moved north to Crimea, leaving the tragedy of his wife's death behind, where he founded the Greil Mercenaries.