"Days With My Stepsister" centers on the evolving relationship between two teenagers, Yuuta Asamura and Saki Ayase, who become stepsiblings after their parents remarry. The narrative begins with Yuuta, a reserved and pragmatic high school student, adjusting to his new living situation. Saki, his new stepsister, is initially distant and aloof, maintaining a polite but cold demeanor. Both characters are navigating the complexities of blending their families while dealing with their own personal struggles and insecurities.

The story unfolds in a slice-of-life format, focusing on the day-to-day interactions between Yuuta and Saki as they gradually grow closer. Their relationship is marked by a slow but steady progression from awkwardness and formality to mutual understanding and emotional connection. The manga explores themes of family, identity, and the challenges of adolescence, portraying the characters' internal conflicts and growth with subtlety and realism.

Yuuta’s calm and thoughtful nature contrasts with Saki’s guarded personality, creating a dynamic that evolves as they learn to trust and rely on each other. The narrative avoids melodrama, instead emphasizing the quiet moments and small gestures that define their bond. Secondary characters, including their parents and friends, provide additional context and depth to the story, highlighting the broader impact of their family’s new dynamic.

The plot avoids overt romantic tropes, instead focusing on the emotional and psychological aspects of their relationship. The pacing is deliberate, allowing the characters’ development to feel natural and grounded. Through its exploration of familial and personal connections, "Days With My Stepsister" offers a nuanced portrayal of the complexities of modern relationships and the process of building trust and intimacy in a blended family.