Kayo first appears as a servant in the Sakai household, a setting that serves as the backdrop for the Bakeneko arc that precedes the main Mononoke series. Her role in this household is that of a maid, and she is notably the first person to allow the medicine seller entry, a decision that initiates the chain of events leading to the confrontation with the mononoke. She later reappears as a passenger on a ship in the Umibozu arc, where she is identified as a seamstress traveling towards Edo.

In terms of personality, Kayo is portrayed as practical and observant. During the events on the ship, she is quick to assess the situation and suspect foul play, openly voicing her suspicions about the other passengers, including the medicine seller, regarding the ship being purposely thrown off course. Her direct and somewhat blunt nature is evident when she confronts the medicine seller with her suspicions, a contrast to the other passengers who are more reserved or deceptive. She is also shown to have a deep-seated personal anxiety, confessing that her greatest fear is a future where she is unable to find a husband or bear a child, a fear so profound that it manifests as a vision of her giving birth to a monster.

Motivations for Kayo are rooted in survival and a desire for a stable future. In the Bakeneko arc, her primary motivation is to survive the supernatural threat within the Sakai household, assisting the medicine seller out of pragmatism. By the time of the Umibozu arc, her motivation shifts to a more personal one, revealing a societal and existential anxiety about her prospects in life. Her role in the overarching story is significant as she is one of the few characters to appear in more than one arc, creating a tangible link between the prequel Bakeneko story and the main Mononoke narrative. She acts as a recurring ally to the medicine seller, having previously witnessed his abilities during the Bakeneko incident.

Key relationships are central to her development. Her most notable connection is with the medicine seller. Unlike many who view him with suspicion or fear, Kayo develops a degree of trust in him after working together against the Bakeneko. During their first encounter, their interactions are noted for a certain playfulness and flirtation, which is considered unique in the series. In the household, she is under the authority of a superior named Sato, and she is often paired with a samurai named Odajima, who acts as a burly but responsible protector during the crisis, though Kayo frequently shows more initiative and bravery than him.

Kayo’s character development is most apparent in her transition from a victim of circumstance to an active participant. In the Bakeneko story, she begins as a frightened servant but ultimately contributes to the exorcism by taking charge of sacred tools, showing a hidden resilience. The traumatic events she witnesses, concerning the abused woman Tamaki, profoundly affect her worldview. By the Umibozu arc, her experiences have made her more cynical and suspicious, yet also more open about her personal fears, marking a maturity from her earlier role. While she does not possess supernatural powers, her notable ability is a sharp intuition and a courageous will. She is able to handle the tenbin, a religious tool used in exorcisms that is said to respond to a person's spirit, and she is one of the few characters brave enough to directly confront the mononoke, such as when she throws salt to repel it.