Shizuku Kadomori is one of the Kadomori twins, charged with protecting the mystical Black Gate guarding access to an alternate world of immense power. She deeply resents her guardian role, feeling perpetually overshadowed by her firstborn sister, Kasumi, who leads the protectors. This bitterness fuels her ambition, stemming from a belief in Kasumi's preferential treatment and her own intense desire to seize the ultimate power hidden beyond the gate, viewing the prohibition against using it as an unfair restriction.

Shizuku possesses a determined and ruthless personality, acting decisively to obtain her goals. She seduces her sister's boyfriend, Narifumi Hosokawa, demonstrating her manipulative nature. Skilled at identifying vulnerabilities, she exploits others to gain allies or remove obstacles, strategically assessing who can be used and who must be eliminated. During her high school's culture festival, she seizes a moment of distraction to employ forbidden magic. She deliberately opens the Black Gate, violating her family's sacred oath, and enters the alternate world. Her primary objective is to claim its power for herself, intending to use it to rule Earth and escape Kasumi's shadow.

Within the alternate world, Shizuku forges alliances with native demonic entities, offering her body in exchange for their aid and the power she seeks. This act solidifies her betrayal of her guardianship duties and family legacy. She prepares to confront Kasumi, Narifumi, and the pursuing guardians – fellow students and a teacher – who enter the gate to stop her ambitions and prevent dimensional chaos. Shizuku awaits them, confident in her demonic allies and her path to ultimate power.

Her portrayal emphasizes her actions and personality over physical description. Her motivations remain consistent as the primary antagonist, driven by resentment towards her sister and the desire for the power beyond the Black Gate.