Fidelius Astrium is the king of Evermore, a central kingdom in the world of Ni no Kuni, and serves as a significant character in the story's events. He is the father of Princess Astrid and the younger brother of Sedulus Astrum, a figure whose hidden identity forms the core of the film's conflict. Fidelius is the son of the previous King Astrum and was once part of a destructive war between Evermore and the neighboring kingdom of Beladon, a conflict he deeply regrets. According to his brother, Fidelius was the favored child of their parents. Before the events of the film, he also lost his wife, which profoundly shaped his personal life.

As a ruler, Fidelius is portrayed as a good and just king, genuinely concerned with the welfare of his kingdom and his daughter above all else. His personality is defined by a strong sense of protectiveness, especially regarding his daughter, Astrid. Having lost his wife, he becomes extremely vigilant and caring towards Astrid, making her safety his primary motivation. When Astrid falls victim to a curse that inexplicably bypasses her Royal Aegis protection spell, Fidelius is shown to be desperately worried and suffering from exhaustion due to the immense stress he is under. This concern makes him initially skeptical of outsiders, such as the protagonists Yu and Haru, and this wariness is exacerbated by the subtle manipulations of his Magic Minister, Gnauss Wisden.

Fidelius's role in the story is primarily that of a guardian and a ruler whose kingdom is under a hidden threat. He is first seen by the bedside of his cursed daughter, desperate for a cure. He serves as an obstacle of sorts for Yu and Haru, as his suspicion of them, fueled by Gnauss, leads to tests of their character, such as arranging a mock swordplay battle against them in the coliseum under the guise of an exhibition. His primary narrative function is to be a well-intentioned king deceived by a treacherous advisor. The truth is eventually revealed that his trusted minister, Gnauss, is actually a disguise used by his own elder brother, Sedulus, who is also leading a rebel force known as the Black Banner under the name Galeroth Fellgrimm. Sedulus's villainy stems from a deep-seated jealousy of being less favored than Fidelius and a desire for revenge following the destruction of Beladon, where he had found a new family.

Following the defeat of his treacherous brother, Fidelius undergoes a development in perspective. He re-evaluates the protagonists who helped save his kingdom and his daughter. In recognition of their deeds, he performs a significant act of gratitude and trust by formally crowning Yu as a Prince of Evermore, solidifying his bond with the visitors from another world. This act demonstrates his capacity to move past his initial skepticism and reward genuine heroism. In terms of notable abilities, there are no specific combat or magical powers detailed for Fidelius himself. His authority is derived from his position as king, and his most significant known asset is the royal lineage, exemplified by the Royal Aegis spell that was meant to protect his daughter. Fidelius Astrum ultimately represents the archetype of the good king, one whose loving and trusting nature is exploited by a resentful family member, but who ultimately proves wise and just in his final judgments.