Mikagi, whose full name is Shiso Mikagi, is a man from the Jomon period of Japan and the founder of the Mikage family. He lived centuries ago and is the original ancestor whose spirit later manifests in the modern-day story. In ancient legend, he stole the celestial robe, or hagoromo, of the celestial maiden Ceres and hid it in the ocean, forcing her to become mortal and remain on Earth as his wife. This act set in motion the central conflict of the series, as Ceres' vengeful spirit seeks to reclaim her robe and destroy the Mikage bloodline.

In terms of personality, Mikagi is initially portrayed as a gentle and kind young man. Ceres falls in love with him, and she grants him power after their family is attacked and he feels ashamed of his inability to protect them. However, as he grows stronger, his love transforms into an all-consuming obsession. He becomes increasingly possessive and paranoid, ultimately going mad and hiding the hagoromo to ensure Ceres can never leave him. His motivations are rooted in a desperate need to control Ceres and keep her bound to him, even at the cost of her freedom and happiness. He stops at nothing to ensure Ceres remains his, prioritizing his own desires over the well-being of his family.

Within the story, Mikagi serves as the primary antagonistic force, especially through his possession of his descendant Aki Mikage, the twin brother of the protagonist Aya. Aki is gradually taken over by Mikagi's vengeful spirit, making him a direct threat to Aya and Ceres. Mikagi's role is that of a tragic villain whose initial love curdles into destructive fixation. His presence drives much of the plot, as Ceres' quest for the robe and her desire for revenge are directly tied to his actions from the past.

Key relationships define Mikagi's character. His relationship with Ceres is the most significant: what begins as a genuine, loving partnership eventually fractures under his jealousy and possessiveness. Ceres comes to fear for the safety of her children, and their bond turns into a bitter conflict. He is also connected to Aki Mikage, whose body becomes a vessel for his spirit in the present. Aki struggles against Mikagi's influence, and their internal battle becomes a major element of the narrative. Another relevant relationship is with his own child, whom he kills in a fit of rage when Ceres attempts to leave him, an act that seals his fate and leads to his death at Ceres' hands.

Mikagi undergoes a tragic development arc. He transforms from a humble, loving man into a possessive tyrant driven by fear of loss. His descent into madness is gradual but complete, and by the end he is wholly consumed by his obsession. In the present-day timeline, even after his death, his spirit continues to seek control over Ceres through Aki, demonstrating that his fixation has not diminished. Ultimately, Aki manages to resist Mikagi's hold long enough to sacrifice himself to save Aya, finally breaking the cycle of obsession and revenge.

Notable abilities associated with Mikagi are tied to his status as the founder of the Mikage family. As a spirit, he can possess his descendants, most prominently Aki, granting him access to their bodies and some measure of their strength. The lacerations that appear on Aki when Mikagi manifests are identical to those that Ceres inflicted on Mikagi when she killed him, serving as a physical mark of his violent past. While Mikagi himself does not display the celestial powers of Ceres, his influence allows him to exert control and continue his pursuit across generations.