Cynthia Landlavizar is a young girl whose childhood exploitation, stemming from exceptional intellect, caused severe psychological trauma. This trauma manifested as dissociative identity disorder, creating two distinct personalities sharing one body. Her primary personality, Cynthia, is present during daytime hours. She is nonverbal, communicating through breathing sounds and physical gestures, and exhibits an innocent, childlike demeanor. Cynthia displays attachment to Taro Hanaukyo, often accompanying him and guiding him through the mansion.

Her alternate personality, Grace, emerges exclusively when Cynthia sleeps. Grace possesses markedly different traits: she is vocal, assertive, stubborn, and adopts a protective stance. Physically, Grace tucks Cynthia's long blonde hair under a cap. As head of the Hanaukyo mansion's Computer Security Department, Grace designed and maintains the MEMOL supercomputer, which manages the mansion's critical systems. She often works through the night on security protocols and maintenance.

The two personalities maintain awareness of each other and communicate internally, though they typically alternate control without overlapping consciousness. This arrangement strains their shared physical body, particularly due to Grace's tendency toward overwork. Grace's self-imposed protective role allowed Cynthia a more carefree childhood but took a physical toll, exacerbating their condition. Key developmental moments occur when Taro discovers Grace during her nighttime patrols, leading to greater understanding of their shared existence. A significant interaction involves Taro comforting Grace during a crisis about their merged identity, helping alleviate her distress.

Their storyline explores the ongoing management of their co-existence and the physical toll of sustaining two identities, implying their situation requires ongoing attention and care within the mansion's environment.