"Onee-chan no Midori-kun" is a manga series that explores themes of family dynamics, personal growth, and the complexities of relationships. The story centers on Midori, a young man who has spent most of his life living with his mother after his parents' divorce. When his mother remarries, Midori is introduced to his new stepfamily, including his stepsister, Aoi. Aoi is a confident and assertive individual who quickly takes on a protective and somewhat domineering role in Midori's life.

The narrative delves into the evolving relationship between Midori and Aoi as they navigate their new family structure. Midori, who is initially reserved and unsure of his place in the household, begins to open up under Aoi's influence. Aoi, on the other hand, struggles with her own feelings of responsibility and the pressure to maintain a strong facade. Their interactions are marked by a blend of humor, tension, and moments of genuine connection, as they both learn to adapt to their new circumstances.

Supporting characters, such as Midori's mother and stepfather, play significant roles in shaping the household dynamics. The series also explores Midori's interactions with his peers and the challenges he faces in balancing his personal life with his academic responsibilities. Themes of self-discovery and the importance of communication are woven throughout the story, as Midori and Aoi work through their individual insecurities and misunderstandings.

The plot progresses through a series of episodic encounters and longer arcs that highlight the characters' growth and the deepening of their relationships. The manga balances lighthearted moments with more serious reflections on family and identity, creating a narrative that is both engaging and thought-provoking. The characters' development is central to the story, with Midori and Aoi's evolving bond serving as the emotional core of the series.