"Moeru! Oniisan" centers on the life of Tatsuya Onii, a high school student who unexpectedly becomes the guardian of his younger sister, Chiharu, after their parents pass away. Tatsuya is determined to provide a stable and loving environment for Chiharu, but his efforts are complicated by his own insecurities and the challenges of balancing school, work, and parenting. The series explores their day-to-day struggles as they navigate their new reality, highlighting the bond between siblings and the sacrifices required to maintain it.

Chiharu, a cheerful and energetic elementary schooler, often relies on Tatsuya for emotional support and guidance. Despite her young age, she is perceptive and occasionally takes on a caretaker role herself, offering encouragement to her brother when he feels overwhelmed. Their relationship is the emotional core of the story, portraying both the joys and difficulties of sibling dynamics.

The narrative also introduces a supporting cast of characters who influence Tatsuya and Chiharu’s lives. These include Tatsuya’s classmates, who provide comic relief and occasional advice, as well as neighbors and teachers who offer assistance or create additional challenges. The series balances lighthearted moments with more serious themes, such as grief, responsibility, and the importance of family.

As the story progresses, Tatsuya’s growth as a guardian becomes evident. He learns to manage his time more effectively, confronts his fears of inadequacy, and gradually becomes more confident in his role. Chiharu, too, matures, gaining a deeper understanding of her brother’s efforts and the complexities of their situation. The series concludes with a sense of hope, emphasizing the resilience of their bond and their ability to face future challenges together.