Kazune Yuki is the elder brother of the main protagonist, Aine Yuki. He is a member of the large Yuki family, which includes several siblings, and he is recognized as the second child and eldest son in the household. Kazune made his first appearance in the very first episode of the series.

In terms of appearance, Kazune has short, slightly shaggy auburn hair that features layered bangs brushed in various directions, along with two small cowlicks. His eyes are a dark yellow color. His personality is presented as fairly shy, especially when compared to the more outgoing nature exhibited by other members of his family. He notably does not participate in the family catchphrase that his other siblings engage in.

Kazune is deeply defined by his caring and protective relationship with his younger sister, Aine. He worries about her and is shown to be emotionally invested in her activities, even tearing up after watching her perform in a television drama. His concern for Aine extends to a slightly jealous or possessive streak; he becomes envious when Aine acts friendly with her close friend and partner, Mio, and recalls a childhood promise where Aine said she would marry her brother.

Kazune's role in the story is primarily as a supportive family member in Aine's life. His motivations center on watching over his sister and her career, demonstrating a strong sibling bond. He is also shown observing Aine through a telescope, remarking on how pretty her dress is, which further illustrates his admiring and watchful nature. While he does not actively participate in the world of idols himself, his presence serves to ground Aine in her family life and provide moments of personal motivation and emotional support.