Zavilia is a black dragon, one of the three great beasts renowned for his immense power and distinctive aura. His first encounter with Fia Ruud occurs when she saves him from a knight brigade trap, forging a familiar bond that establishes him as her devoted protector. He accompanies her into the First Knight Brigade, concealing his true nature by taking a human form disguised as the weakest monster type. Despite this disguise, the sheer intensity of his aura constantly risks exposure.

As Fia's companion, Zavilia exhibits unwavering loyalty, prioritizing her safety above all else. When Fia faces a lethal assault from blue dragons during a forest patrol, he shreds his disguise to shield her, revealing his true adult black dragon form. This act confirms his powerful guardian status and willingness to sacrifice their secrecy for her survival. Later, Zavilia departs on a personal journey to ascend as the Dragon King, temporarily leaving Fia's side. His absence affects her deeply, though their bond endures. After achieving his goal, he reunites with Fia, resuming his protective role.

Zavilia's development expands his relationships beyond Fia, including interactions with dragons on Blackpeak Mountain. His journey toward kingship signifies growth from a solitary guardian to a leader among his kind, yet his commitment to Fia's well-being remains steadfast throughout these changes.