Fidelius Astrum, King of Evermore and son of a previous King Astrum, was the younger brother of Sedulus Astrum. Sedulus claimed Fidelius received their parents' greatest favor. Fidelius fought in the historic battle between Evermore and Beladon, an event that left him haunted by lingering regrets.

Before the movie's events, his wife's death drove him to become intensely protective of their daughter, Astrid Astrum. He manifested this protectiveness through the Royal Aegis spell, shielding Astrid from physical harm. When Astrid succumbed to a curse despite this protection, Fidelius plunged into significant distress and exhaustion fueled by profound worry over her condition.

He initially greeted the arriving Yu and Haru with skepticism, an attitude amplified by his court mage, Gnauss Wisden. Later, during an attack on Evermore, Gnauss was exposed as a traitor. Further investigation revealed Gnauss as Galeroth Fellgrimm, leader of the Black Banner, and ultimately unmasked him as Fidelius's older brother, Crown Prince Sedulus, operating under a false identity.

Following Sedulus's defeat, Fidelius formally recognized Yu's contributions by crowning him a Prince of Evermore, solidifying Yu's status within the kingdom.