"Asahina Wakaba to Marumaruna Kareshi" centers on Asahina Wakaba, a high school girl who struggles with social anxiety and self-esteem issues. Her life takes an unexpected turn when she encounters a mysterious, round, fluffy creature named Maru, who claims to be her "boyfriend." Maru is a supernatural being with the ability to transform into a human boy, though his human form retains some of his fluffy, round characteristics. Despite his peculiar appearance and behavior, Maru is deeply devoted to Wakaba and seeks to help her overcome her insecurities.

The narrative explores Wakaba's gradual growth as she navigates her relationship with Maru, who consistently supports her with unwavering kindness and encouragement. Maru's presence challenges Wakaba to confront her fears and step out of her comfort zone, often leading to humorous and heartwarming situations. The series balances lighthearted comedy with moments of emotional depth, particularly as Wakaba begins to open up to others and form connections beyond her relationship with Maru.

Supporting characters include Wakaba's classmates and friends, who add layers to the story by providing different perspectives on her journey. Some characters are initially skeptical of Maru's existence or intentions, while others quickly accept him as part of Wakaba's life. The dynamics between Wakaba, Maru, and the people around them create a blend of slice-of-life storytelling and supernatural elements, emphasizing themes of self-acceptance, personal growth, and the transformative power of love and friendship.

The plot progresses through episodic encounters and challenges, with each chapter highlighting Wakaba's evolving confidence and Maru's endearing efforts to understand human emotions and relationships. The series maintains a consistent tone of warmth and optimism, making it a comforting read that resonates with themes of overcoming adversity and finding joy in unexpected places.