"Idol Sister" centers on the life of a young man named Haruto, whose ordinary routine is disrupted when his long-lost sister, Ayaka, suddenly reenters his life. Ayaka, now a famous idol, returns home after years of separation, bringing with her the glamour and chaos of the entertainment industry. Haruto, who had grown accustomed to a quiet and solitary existence, finds himself thrust into the spotlight as Ayaka insists on rekindling their sibling bond. The narrative explores their evolving relationship as they navigate the challenges of fame, family, and personal identity.

The story delves into the complexities of Ayaka’s dual life as both a beloved public figure and a sister seeking to reconnect with her brother. Her idol career demands perfection and constant public scrutiny, creating tension as she struggles to balance her professional obligations with her desire for a genuine familial connection. Haruto, on the other hand, is initially reluctant to embrace the changes Ayaka brings, wary of the attention and disruption to his routine. Over time, however, he begins to understand the pressures Ayaka faces and the loneliness she hides behind her cheerful public persona.

Supporting characters include Ayaka’s manager, who serves as a mediator between her career and personal life, and Haruto’s close friends, who provide comic relief and emotional support. The series also introduces rival idols and industry figures who add layers of conflict and competition, further complicating Ayaka’s journey. Themes of perseverance, self-discovery, and the importance of family are woven throughout the narrative, as both siblings learn to navigate their shared past and uncertain future.

The plot is driven by a mix of heartfelt moments and lighthearted humor, with occasional dramatic twists that highlight the challenges of life in the public eye. The dynamic between Haruto and Ayaka forms the emotional core of the story, as they gradually rebuild their relationship while confronting external pressures and internal doubts. "Idol Sister" balances its exploration of the idol industry with a focus on personal growth and the enduring bond between siblings.