Duke Aegir is the latest in a long line of Dukes of Aegir to inherit the position of Prime Minister of the Adrestian Empire. In Imperial Year 1171, he masterminded the Insurrection of the Seven, a soft coup that stripped Emperor Ionius IX of much of his power and reduced him to a figurehead. Since then, Duke Aegir has effectively ruled the Empire from behind the throne and worked closely with Those who Slither in the Dark, performing experiments on the Emperor's children.
When Edelgard is made Emperor, one of her first decrees is to strip him of his titles, Duke Aegir tries to argue before backing down. During the War Phase (not in Crimson Flower), in Ferdinand and Lysithea's Paralogue, Duke Aegir escapes from house arrest and winds up in Hrym territory. Confronted by people who have experienced atrocities by Aegir's rule, along with Lord Arundel's actions in Aegir's name, Duke Aegir is killed.
Noticeably corrupt, Duke Aegir's attitude is the exact opposite of his son, and likely the main reason why Ferdinand tries to act as a proper noble.
- Duke Aegir shares his English voice actor with Gilbert as well as Mycen from Fire Emblem Echoes: Shadows of Valentia.