Kō Hayama is the elder sister of Teru Hayama and a supporting character in the anime. She is a high school student with long blonde hair and olive-colored eyes, born on April 21. Her personality is generally kind, gentle, and calm, with an innocent and somewhat spacey demeanor. This innocent nature sometimes causes her to unintentionally say or do things that can be hurtful to others, though she never means any harm. She is deeply fond of animals and owns a pet cat named Beelzebub, whom she affectionately calls Bel.

She has a notable trait of being extremely lucky, frequently winning lotteries and contests, and she often shares her winnings with those around her. However, she is also famous for her disastrous cooking skills. She insists on preparing food that she claims is healthy, but the results are consistently terrible in both taste and smell, and others find them nearly inedible. Despite this, she remains sincere in her efforts to be helpful.

In the story, Kō appears as a recurring background figure who interacts with the main trio—Yoko Nishikawa, Futaba Odagiri, and her sister Teru—during school events and casual gatherings. She often provides comic relief through her lucky finds and her stubborn commitment to her awful cooking. Her relationship with her younger sister Teru highlights a contrast: while Teru can be sharp-tongued and direct, Kō is gentle and oblivious.

Kō does not undergo significant personal development over the course of the series, instead serving as a consistent, light-hearted presence. Her defining abilities are her extraordinary luck in games of chance and her uniquely poor culinary creations, both of which are used to generate humorous situations.